When Ewan McGregor was but a wee lad in a sleepy town in Scotland in the ’70s, some 3,200 miles away in a Manhattan townhouse on East 63rd Street, Roy Halston Frowick was living on a diet of baked potatoes with beluga caviar, chilled Stolichnaya, rent boys and mounds of cocaine piled in Elsa Peretti silver ashtrays. (With silver straws to match.)

“I’d never heard of him,” said a bearded Mr. McGregor, Zooming from his L.A. home. “I didn’t know Halston at all.”

Then the actor got cast as the man with one name who lost the rights to that name, the man who was at the center of a wild and tragic era in New York, the man who is known as America’s first celebrity fashion designer.

And Mr. McGregor, now 50, became so possessed with Halston in the eponymous Ryan Murphy mini-series coming to Netflix that sometimes he thought he was Halston, that he was the one who designed Jackie Kennedy’s pillbox hat for the inauguration.

“There were just little moments,” Mr. McGregor said, “where I felt like, ‘Oh, that was him.’ There was, like, a little curse, an eye roll or something where I felt that was it.”

It’s a tricky role since he has to play someone playing someone. Like Gatsby, Halston, who grew up in Indiana, created himself. He slicked back his hair, slathered on Guerlain bronzer, donned dark sunglasses, pulled on a black turtleneck and adopted a Continental locution.

He would be gossiping and laughing with his assistant Tom Fallon as they walked to work when Halston was the custom milliner at Bergdorf’s early in his career. Then, as soon as they pushed through the revolving doors, the voice and mien changed. “Hahl-ston had arrived,” Mr. Fallon recalled in a documentary.

No matter how famous the designer got, no matter how supercilious and sleek he seemed, he never lost that sense of being an outsider. In the show, “H,” as he is known, calls his inner circle “a bit like little ships lost at sea … a bunch of queers and freaks and girls who haven’t grown up yet.”

Matt Tyrnauer, who made a documentary about Studio 54, compared Halston to Cole Porter, also from Indiana. “Two Hoosiers who took Manhattan and defined cafe society in their respective generations,” he said. “Both pretty out there with their sexual proclivities and both emblematic of the need for gays to flee and realize their cosmopolitan dreams.”

Mr. Murphy’s show explores Halston’s volatile relationships with Victor Hugo, an escort who became his longtime companion, and Peretti, his model and jewelry designer who was, futilely, in love with him.

The 5-foot-10 Mr. McGregor willed himself to elongate into Halston, whose massive ego and regal bearing made him seem taller than his 6 foot 2 inches — with some expert help from the costume designer and cinematographer.

“I was always upset that I didn’t have his hands because he’s got beautiful, long, elegant fingers and I don’t,” he said. “When I’m doing the scenes pinning and draping, I was slightly upset that I wasn’t able to act his hands.”

Mr. McGregor is now in a galaxy far, far away making his Obi-Wan Kenobi TV series for Disney, but he is haunted by Halston. “I loved being him. It was so much fun.”

Not quite as much fun as the real Halston, since Mr. McGregor was smoking green-tea cigarettes instead of Trues, snorting sugary, vitamin-like Inositol instead of Bolivian marching powder and, due to Covid restrictions, conjuring disco fever with just 48 extras in the Studio 54 scenes.

“The part of me that’s a smoker thought, OK, great, I can smoke again while I play Halston,” he said, with his raffish grin. But then, “The green tea cigarettes are so horrible.”

‘A New Level of Cool’

When those who knew Halston in his heyday heard that Mr. McGregor was cast, they raised their manicured eyebrows. They could not picture the rough-and-tumble man-boy who likes to ride motorcycles and wield lightsabers as the grander-than-thou designer, whipping up tie-dye caftans, buttering up society matrons with a buffet of Ultrasuede wrap dresses, dispatching a plane from Manhattan to Montauk with his seafood lunch.

But when they watched screeners, they felt as though they were actually watching Halston.

“That Scottish man pulled it off,” said André Leon Talley, the author of “The Chiffon Trenches.” “The way he pivots with the cigarette dangling between his two fingers and says ‘Bah-len-ciah-aaaaa-gah,’ so it’s almost musical with the ‘B’ floating on air, it was so perfect.”

“I cried,” said Mr. Talley, who was working at Andy Warhol’s Interview in those days and was a regular at Studio 54 and Halston’s ultrachic gray lair, observing the Dionysian scene. “The story tells you the rise of the American dream and the ruin of the American dream. He was like Cary Grant, one of the greatest talents, up there with Saint Laurent. He dressed Jackie Kennedy, Lee Radziwill, Lauren Bacall. Then he flew too close to the sun, the Icarus myth. Sex and drugs and promiscuity ruined him, that Sodom and Gomorrah in the balcony and the basement of Studio 54. That Venezuelan call boy was his downfall.”

He was referring to Victor Hugo, who like Halston invented himself and gave himself a new name, a play on his anatomy. “Believe me, he wasn’t reading French literature,” Mr. Talley said. “He was a grifter who clung on like a parasite, like a barnacle on a ship.”

In one scene, Halston brings Hugo’s jock strap to a perfumer as an inspiration for his first perfume (with bottle designed by Peretti).

Some, like Warhol and Anjelica Huston, enjoyed the bravura of Hugo, who called himself Halston’s “art adviser” and designed Halston’s shop windows eclectically as a crime scene or a mannequin in labor.

“I thought he was great fun,” said Ms. Huston, who was one of the “Halstonettes,” as the designer’s entourage of models was called. “There was a fizziness about the moment, like bubbles in the Champagne. We all played off each other. Halston put those Grace Kelly clothes on those Black girls and he struck a note that was fantastic. It was a new level of cool.”

Tom Ford, who said he was “a twinkie” at Studio 54 in the days when Halston hung out there with Liza Minnelli, Liz Taylor, Bianca Jagger and Warhol, was also impressed with Mr. McGregor and the show.

When Mr. Ford was 18, a date took him to a gathering at Halston’s house, which inspired him. “It’s minimalist,” he said, “but it’s sensual and sexy.” Four decades later, he bought the house, and he’s now restoring it in the Halston image, reproducing the gray Paul Rudolph furniture on the bottom three floors and turning the top floor into a replica of Halston’s ripe-cherry red office in the Olympic Tower.

“I really wish those walls could talk, or turn into video screens,” Mr. Ford said. “I wish I could see what happened in that house.”

He said there was an old picture of Hugo peeing into a toilet from a bathroom which will now be used by his son, Jack.

“I don’t think it would be appropriate to hang that picture over Jack’s toilet,” Mr. Ford said dryly.

Valerie Steele, the director and chief curator of the museum at the Fashion Institute of Technology, said she believes Halston’s excess undermined his success in the fashion pantheon.

“I think in a way we’ve been misled by the chaos of the Studio 54 scene to not think as highly of Halston as we should,” she said. “I think he was one of the great designers.” (Indeed, he will be featured in both parts of the Met’s upcoming Costume Institute exhibit on American fashion.)

There are more amazing Halston stories than the series has room for: Ali MacGraw told me about buying her wardrobe from the designer to promote “Love Story” in Paris and London. She recalled getting ready before a charity gala for the queen and realizing that she couldn’t get her black Halston slip dress to fit correctly.

“No matter how I turned it, my breast was hanging out,” she recalled, saying she was “flabbergasted.” Panic mounting, the car downstairs, she wore the only other clothing she had with her, some black satin pants (which did not pass muster with the British tabloid reporters). Her then-husband, Bob Evans, was upset and called Halston. Back in New York, Ms. MacGraw was summoned to the salon. Flanked by two assistants, he languidly instructed her to put on the dress and “turn it around.”

“And, of course, it was perfect,” Ms. MacGraw said. “It was mortifying. I sent him many dozen red and yellow tulips with a card that said, naturally, ‘Love means never having to say you’re sorry.’”

Ms. Huston told me about going to Halston’s house for dinner when she was visiting New York, six months after she had moved to Hollywood. She was wearing an imitation Halston dress she had bought in a shop on Sunset Boulevard. “I walked in his door and he said, ‘Oh my God!’ and sent one of his boys upstairs to get a roll of fabric which he wrapped around me for dinner,” she recalled. “I was so embarrassed.”

Snarls for Calvin

Mr. McGregor prepared for the role by having tea with Ms. Minnelli, who was Halston’s best friend, and obsessively watching “Project Runway.” He also worked with the show’s costume designer, Jeriana San Juan, to learn draping, pinning and cutting fabric. Since Halston scorned undergarments on his models, pinning was not without peril.

“I got a sewing machine,’’ he said. “I did a bit of sewing and I had, what do they call it? The dummy? I had one of those. I had some fabric. I bought some books.”

He tried making himself some pants. “I looked at trousers that I owned,” he said. “I was turning them inside out and just trying to figure it out.” He made the first pocket and was really impressed with himself. But then when he put in the second pocket, he realized one was on the outside and one was on the inside.

“They were cool. There was something Japanese about them. Very now, I thought.” But he used such a heavy blue wool, they were too itchy to keep.

Unlike his character, who strides around in long, double-faced wool red, cream and black trench coats, red pants and cashmere cardigans, the actor dresses in daily life with casual cool.

“Ewan has a great sense of fashion, he’s really stylish,” said Baz Luhrmann, who directed Mr. McGregor in 2001’s “Moulin Rouge!”, a time when the actor was “at his most wild.”

Asked to describe what he is wearing on Zoom, Mr. McGregor pulled off his blue sweater to check the tag on his T-shirt, showing the top of his Calvin Klein underwear over his pants.

It was a funny moment, given the contempt Halston felt for the younger rival nipping at his heels. In one scene in the series, Halston calls Calvin Klein a “hack” and sneers, “There’s something so Long Island about the whole thing.” In another, he throws a glass full of scotch at the television screen when Brooke Shields comes on to purr, “You want to know what comes between me and my Calvins?”

Mr. McGregor laughed at his impromptu fashion commentary. “I’m wearing Buck Mason straight-cut T-shirt in stripes,” he said. “I’m wearing some indoor-outdoor slippers from huckberry.com. That is an amazing website. These trousers I have no idea where I got these from but they are like a drawstring pant. They’ve got no pocket and they’re made of canvas.”

His two oldest kids — he has four daughters with his former wife, Eve Mavrakis, a production designer — are following in his stylish footsteps. Clara, 25, and Esther, 19, starred in a Fendi ad together in 2018. Clara is a Wilhelmina model.

“They are beautiful, aren’t they?” Mr. McGregor said. “I was able to visit them on a shoot that they both did together in New York.” After so many years where they tagged along with him, he said, it was “cool” to be the visitor in their world. “I actually went out and got them some sandwiches. There was no catering.”

Mr. McGregor’s home life became press fodder a few years back when he and Ms. Mavrakis split after 22 years of marriage.

The actor is now in a relationship with Mary Elizabeth Winstead, his co-star in season three of “Fargo.” His role playing both Stussy brothers earned him a Golden Globe in 2018.

Clara and Esther both expressed their dismay about the split on social media. Clara also wrote an Instagram post in 2019 about going to rehab to overcome a Xanax addiction, which has turned into a movie project, with her father producing, according to The Hollywood Reporter.

I asked Mr. McGregor how his own experiences with addiction factored into his portrayal as Halston.

“I’ve been sober a long time and I’m interested in it,” he said. “It’s part of my life in terms of sobriety and having struggled with addiction.”

Was his problem with alcohol exacerbated by drugs?

“It was the ’90s,” he said with a dry smile. “It was lots of things.”

He was feeling so “miserable” by 2001 that he gave up alcohol and, for good measure, cigarettes. He misses the cigarettes more.

“At the time, I thought it was cool and made me cool and makes you feel tough and hard,” he said. “Actually, who I am isn’t that. That’s just not who I was.” He said his ex-wife helped him and other people talked to him about his behavior.

“I wasn’t getting away with it like I thought I was,” he said, adding: “It’s like somebody living inside you which is hellbent on destroying you, really. That’s what it wants.”

In the show, the Liza character, played by Krysta Rodriguez, tries to get Halston to come to the Betty Ford Clinic with her, telling him gently, “The one thing you don’t know how to do is stop.”

The actor had to relive all that tracing Halston’s spiral. (The show, directed by Daniel Minahan, sprang from the Steven Gaines biography, “Simply Halston.”)

When Halston’s secretary, Sassy Johnson, tells him he has gone through two weeks’ worth of cocaine in a day, he screams at her to refill the Peretti dish. “GET IT, SASSY!” he shrieks.

As he loses control, the designer begins lashing out at those closest to him as he realizes he can’t oscillate between high (Bergdorf) and low (JC Penney). Eventually he is taken over by the corporate parent of Playtex, which spurred him to comment in real life, “The folks from Planet Tampon landed on Planet Halston.” (The brand has gone through many more iterations, including a stint when Harvey Weinstein and Sarah Jessica Parker were involved.)

Mr. McGregor is known for being warm, cheerful and collegial on sets. Yet he said he could understand Halston’s outbursts with the suits — “the loggerhead between art and commerce” — as the designer got pushed, and seduced by greed, into making a plethora of products that diluted his brand.

“I had somebody once tell me they wanted me to shout this line that I wasn’t shouting, a producer,’’ the actor recalled. “I was, first of all, absolutely appalled that a producer was giving me a note on set and so I just calmly asked why. He said, ‘We think it could be a great trailer moment.’ I said, ‘OK.’ Then we went to the next take and I made this line so quiet that you couldn’t hear.” He snapped at the producer, “I’m not here to make your trailer moments — that’s not my job.”

Mr. McGregor had no problem with the bold sex scenes in “Halston.” He has always embraced onscreen intimacy with men and women, and never shied away from nudity. But he said it got boring for a long stretch when journalists only asked him about being sober and being naked, something he labeled tabloid trash, with an expletive. It reached peak cringe in 2005 when Esquire referred to his penis variously as “man-root,” “light saber,” “Scottish sausage,” and an “anaconda” — all in the same profile.

“Well, it’s a shame,” he said. “There’s a lot more to my life than that. I put up with it. Now it doesn’t happen so much anymore.”

Because they filmed during the height of Covid in Manhattan, the sex scenes were preceded with a lot of hand sanitizer, mouthwash and spitting into buckets.

They also had an intimacy coordinator, which Mr. McGregor felt was an especially important development for young actresses.

“Imagine you’re 22 and you just got cast and you’re on set,” he said. “The leading man is always 25 years older than you, and you’re suddenly with a male director and a male leading actor. Everyone’s always embarrassed about it. It’s like, ‘Just see what happens.’ This poor 22-year-old girl — maybe she doesn’t want what happens to happen.”

With the intimacy coordinator, he said, “It’s literally like, ‘Are you happy to have your bum touched?’ ‘Are you happy for somebody to put their hand on your chest?’ ‘Are you going to kiss with tongues?’”

Last year, there was backlash to James Corden’s campy performance in another Ryan Murphy production, “The Prom,” with some condemning it as homophobic. And certainly, in less skilled hands, the portrayal of Halston — who had an intriguing blend of masculine and feminine qualities — easily could have gone off the rails.

I wondered if Mr. McGregor had worried about the argument that straight actors shouldn’t play gay roles.

“I haven’t walked in the shoes of somebody who’s had to hide their sexuality for fear of not getting roles, and I respect those that have,” Mr. McGregor said. “I will always have that argument with myself, and then with the director and the producers, before choosing to do something. It’s not like I don’t care. It’s not like I think it’s silly. I don’t. I do understand and I do get it. I hope people aren’t offended.”

Back in 1999, when he began playing Obi-Wan in the “Star Wars” prequels, Mr. McGregor readied himself to go from indie darling to big-budget star. But, as he said, “Those three films were pretty universally underwhelmingly received. People didn’t very much like them at the time. It was quite difficult to decide to do something that big and then to get involved in it and then to struggle through some of it and then for people not very much to like it. It wasn’t the experience I thought it was going to be, I suppose.

He remembered being first shown around the props department and sets by George Lucas. “It blew my mind. I was young and had just done ‘Trainspotting,’ and I believed in myself as a grungy, gritty urban actor, and there I was deciding to do one of the biggest franchises in the world.”

Walking on set with Mr. Lucas, he observed the making of a submarine for the film. “I looked at it, all excited, and said, ‘Oh my God, will we go underwater?’ And George just looked at me and laughed and went, ‘It’s not even real, you know.’”

The actor found the green-screen experience grueling and was openly dismissive about a job that mostly entailed perfecting the Jedi frown and waving a saber. “There’s not a great deal of emotional excavating to do in the scene and it can become very hard work,” he said.

He was Zooming from the bedroom of his daughter Anouk, who’s 10. He walked over to the bureau and picked up an Obi-Wan doll.

“That’s me,” he said wistfully. “It’s never been touched. It’s never moved. It’s never been played with. I don’t know what their experience is of having a dad who’s Obi-Wan Kenobi. I’m sure it can be pretty tedious in school and stuff. There’s no discussion about ‘Star Wars.’ I don’t think they’ve ever watched them. They’re really not interested.

“I think it’s complicated for kids whose parents are famous or actors or whatever. It’s probably quite cool and quite tedious at the same time.”

Despite his “up-and-down experience” with the movies, he got in touch with Disney and said, “Look, just so you know, if you ever think to do a spinoff with my character, I’d be up for it.”

Now, he sounds excited about the show he’s doing for Disney Plus. He thinks his director, Deborah Chow, who directed episodes of “The Mandalorian,” is brilliant. He’s even ready to go hang out with superfans at conventions.

Still, he’ll miss the dark side on East 63rd Street. “I was sad to know that I was going to have to let him go,” Mr. McGregor said about Halston.

He kept the black trench coat.

Confirm or Deny

Maureen Dowd: Your style icon is Steve McQueen on a Triumph.

Ewan McGregor: I’m a bit bored of that being a style icon, I’m afraid. When you do a photo shoot, they always show you a picture of Steve. I guarantee it. They always go, “We think it’s like this.” Before I look, I know it’s Steve McQueen on the back of a motorbike. It always is. God knows Steve McQueen was cool and he looked great on a bike but come on.

Obi-Wan throws shade better than Halston.

Nobody throws shade like Halston does.

Like Halston, you have a taste for caviar.

I used to. I remember when I was young, it made me feel very fancy and successful when I was in Heathrow Airport flying anywhere. They sold these tiny little tins of caviar in the airport there, and they would put it in the little Styrofoam box with ice and you were able to take it on the plane, and I would love to sit on the airplane with my little tin of caviar and eat it as I flew to my next project.

In the “Who wore it better?” contest, your Halston black turtleneck beats Jennifer Lawrence’s Elizabeth Holmes black turtleneck.

I think it probably does. I would also like to take this opportunity to thank Sandra Bullock because in “Ocean’s 8,” she has her hair always inside her collar. Mary and I and my kids are always like, “We call it the Sandra Bullock.” In the exterior street scenes in the early ’60s as Halston, I always had my long hair inside the collar.

It was your idea to drop your pants in “Velvet Goldmine.”

Yes, confirmed.

You have a He Shed to house the motorcycles that you rode around the world on, logging 49,000 miles, with your buddy Charley Boorman.

I’ve got a beautiful shed that I’ve built in my property with my stuff in it. I moved to America in 2008 and I rented a little hangar at Santa Monica Airport and I had my bikes and cars there. Now I’ve got them here. I’ve got probably 10 to 12 motorcycles and six cars. I’ve got some old Moto Guzzis from the early ’70s, then I’ve got some ’80s bikes, and then I’ve got some modern bikes. I like VWs as well, so I’ve got old VWs.

Halston’s greatest undertaking was giving Liz Taylor advice on her love life.

That was part of his success, that he was such a great girlfriend to all these ladies.

You went to drama school with Daniel Craig.

I think he was two years or a year above me. I remember very clearly, as a young first-year drama student, seeing him in the corridors and his biker jacket and his long, blond hair. He was quite an imposing character.

Your uncle was a rebel pilot in the original three “Star Wars” films.

Confirmed. He played Wedge Antilles.

Darth Maul is a cooler villain than Darth Vader.

No, definitely not. He would like to think so.