For decades the New York theater world paid homage to J. Smith-Cameron, a veteran stage actress who has often been compared to Carole Lombard for her precise timing and comic verve. When she wasn’t doing Molière or Shakespeare, she was impressing critics and her fellow actors with her performances in plays by Paul Rudnick, John Patrick Shanley and Beth Henley.

Now her hard-won local fame has been eclipsed by her breakout role on “Succession,” the HBO drama about a venal Murdoch-like family locked in a “King Lear”-like power struggle. As Gerri Kellman, the long-suffering general counsel and consigliere to Logan Roy, the vicious, vacillating patriarch played by Brian Cox, Ms. Smith-Cameron has turned an ancillary player into a surprisingly complex character. It’s a grown-up role for a grown-up woman.

Gerri’s cool gaze, raised eyebrows and clipped interjections, along with her shrewd analyses of corporate shenanigans, have made her an avatar of female power for women of all ages, especially young professionals who find that attaining success in their fields may require them to tiptoe around monstrous male egos. As a result, Ms. Smith-Cameron has gone from a darling of the stage to social media star, with memes galore and Twitter accounts dedicated to Gerri’s every eye roll.

“Characters like hers are often written as these barracuda businesswomen or hard-boiled lady detectives, people who are impenetrable or invincible,” Ms. Smith-Cameron said. “What I like about Gerri is she’s very powerful, but inside she’s a nervous wreck. She’s not impervious to things. That’s why I think she strikes a note. There’s a vulnerability to her and a jittery, thinking-on-her-feet quality. She’s not just coming in and blasting people.”

On a brisk March afternoon, Ms. Smith-Cameron, who goes by “J.,” settled in with a cup of coffee onto a squashy blue velvet sofa in her living room. Brownie, a grizzled and wary 12-year-old terrier mix, was napping, fitfully, among the pillows, occasionally rousing herself to bark at a guest.

For the last eight years, Ms. Smith-Cameron and her husband, Kenneth Lonergan, the playwright, screenwriter and director, have rented this cozy, two-story apartment in a Federal-style townhouse in downtown Manhattan from the actor Matthew Broderick. Mr. Broderick and Mr. Lonergan have been pals since high school, and they and Ms. Smith-Cameron have worked together, on and off, for decades.

“It’s been thrilling to watch J. cross over from a fixture on the New York stage into the collective consciousness,” Mr. Broderick said in a phone interview. “She’s so smart and her humor is so slyly funny. She doesn’t miss a joke.”

There are a few Broderick touches in the apartment, notably paintings by Mr. Broderick’s mother, Patricia. “This one is called something like, ‘No matter how old or sick they are, no one likes to look a wreck,’” Ms. Smith-Cameron said, pointing out a piece above the fireplace, an expressionist image of a stately woman in disarray. “Isn’t it great? It’s so thought through.”

Gerri has been good to the Smith-Cameron-Lonergan household.

“She’s been supporting us for the last six years,” said Mr. Lonergan, who is known for the films “Manchester by the Sea” and “You Can Count On Me.” “No qualms with her whatsoever. Whatever she needs to get done it’s fine with me.”

He mused about what, if anything, the character has in common with his wife.

“J. has pointed out that Gerri is very anxious,” he said. “J. is sometimes anxious but not in a maneuvering way — she just gets anxious and overwhelmed. Her wheels are always turning. When you hug her, she’s very nicely hugging you back, but you get the sense she’s thinking of other things.”

“I’m sorry,” Ms. Smith-Cameron said.

It was Ms. Smith-Cameron’s rapport with Kieran Culkin, who plays Roman, the youngest, sassiest Roy, that inspired a “Succession” subplot that completely unhinged the internet. Gerri and Roman were in a jousting and affectionate mentor-mentee relationship as she took him under her wing. But the show’s writers, noting the actors’ off-camera banter, pushed the relationship further. (Off the set, the prankish Mr. Culkin teases Ms. Smith-Cameron as relentlessly as Roman does Gerri. This summer, during a cast dinner, she said, she was so exasperated with him she threw a drink in his face.)

Midway through the second season, Roman’s Gerri-baiting and his off-color jokes, and Gerri’s snappy retorts, had morphed into a queasy dominatrix-submissive scenario. During a phone call with Roman, Gerri realizes, to her horror, that her tart insults are turning the conversation into phone sex, at least on his end. Ms. Smith-Cameron found herself improvising, which was how the phrase “little slime puppy,” a put-down she coined on the spot, entered the popular lexicon. Or at least the vernacular of “Succession” fanatics.

By the end of Season 3, things had gone completely off the rails. Roman tried to text Gerri a close-up of his anatomy, only to misfire, sending the photo to his father. For the first time in her career, Gerri found herself in a vulnerable position. That precariousness, and her response to it, will define her path in the show’s fourth and final season, which has its first episode Sunday.

“Gerri is in a restless, insecure place through the whole season, but also, I feel, getting wise to her heft,” Ms. Smith-Cameron said. “I always felt like there was something kind of on the boil with her. I can say that it’s the first time in her career that she’s not felt on solid ground — and she’s angry about it. She’s angry with both Roman and Logan. She’s of an age and has accrued money and could easily retire, but she’s not the type. She’s a workaholic and I think she feels like she’s in her prime. People are always asking me, ‘Why does she take it?’ I think it’s thrilling for her, it’s a high, like surfing in a dangerous sea.

“I don’t know that I could be Gerri in real life, and yet acting is very insecure,” she continued. “You have to go out and kill for food every time.”

Ms. Smith-Cameron, 65, was born Jean Isabel Smith in Louisville, Ky., the youngest of three children, and grew up in Greenville, S.C. Her father was an architect and engineer; her mother worked at Head Start and as an assistant librarian. Ms. Smith-Cameron studied at the Florida State University School of Theater in Tallahassee but dropped out, because she was working so much in regional theater and small films.

She changed her name in stages: First to J. Smith, because Jeannie, as she was known, seemed too flimsy for an actor. She then exchanged Smith for Cameron, a family name, for additional heft. The hyphenate Smith-Cameron came a bit later, and by accident, after a director printed her name on a film poster that way.

In the early 80s, Ms. Smith-Cameron moved to Manhattan and has worked to growing acclaim ever since. In the 1997 Off Broadway production of Douglas Carter Beane’s “As Bees in Honey Drown,” she played an irresistible con artist, delivering a manic mash-up of Auntie Mame and Holly Golightly in a role that won her an Obie. Ben Brantley called her performance “deliriously pleasurable” in his review for The New York Times.

“In my 60s, to have this attention, it’s just weird,” she said. “It’s not like I didn’t have notice before, but I always did these off-the-beaten track things. I wasn’t trying to be on a big hit show.”

Ms. Smith-Cameron has long been a booster of independent film. You can see her right now in “The Year Between,” by Alex Heller, a comedic drama based on the filmmaker’s own experience with bipolar disorder, which caused her to drop out of college and move back home with her parents. Ms. Smith-Cameron plays the tart Midwestern mother, and Steve Buscemi is her kindly husband. It’s not the first time they have been married onscreen. “He’s so great,” she said of Mr. Buscemi. “We both love to champion independent movies because they’re not built on the premise of making money. They’re exhausting, you have to work really hard fast, but when it fits, it’s a joy.”

“J. lifts people up,” said Zoe Winters, another fine stage actor scooped up by the “Succession” team who plays Kerry, Logan Roy’s immaculate assistant. “I’ll get texts from her that say, ‘You’re quite something. You’re dazzling.’ She has an endless capacity for that. Ultimately, I think what she’s always trying to do is make people feel good and make really good art.”

Ms. Smith-Cameron and Mr. Lonergan met cute, as she put it, while working on a series of one-act plays in the mid-90s. She said she found him appealingly grumpy and quietly hilarious.

As she recalled, “I was like, ‘Why have I never met this actor? He’s of an age, he’s really good, he’s really smart! Is he gay? Is he married?’ I began to do a little research.”

She learned he was a playwright, acting in another’s scene, who had also written what she thought was the best play of the program. When they collided one night on the stairs of the theater, she complimented his work, comparing it to a William Inge play. When he looked blank, she challenged him, saying, “Don’t you know who William Inge is?”

“I had been married and divorced in my 20s,” she said, “and I was going through a chilly spell. I didn’t think I’d fall in love or get married or have kids. So I was a little bitter and a little saucy. But I had never been this brazen.”

The couple married in 2000; their daughter, Nellie, is 21.

“There’s something conspiratorial about J., as if she can’t wait to let the world in on the most delicious secret,” said Mr. Rudnick, who cast her in his 1994 Off Broadway comedy, “The Naked Truth.”

“She was helplessly, magnetically funny,” Mr. Rudnick continued. “I kept making this one speech longer, just so I could watch J. perform it. She developed a brilliant set of almost balletic gestures, which she informed me were called ‘puppet hands.’ And over the course of an especially long rehearsal, we developed a system where if J. performed her monologue flawlessly, I’d give her a chocolate chip cookie. She of course ended up with an entire bag of Chips Ahoy!”

Frank Rich, the former New York Times theater critic and an executive producer of “Succession,” said he had taken delight in Ms. Smith-Cameron’s stage work for years. Even though she has often been known for playing more flamboyant characters, he is not surprised by the nuanced quality she has brought to her character.

“For Gerri, J. found this astringent comic tone that suggests she’s in on the joke of working for these entitled jerks who think they know what they’re doing but often have no idea,” he said. “She’s their corporate babysitter even as she has to be subordinate to them. There’s a tragicomedy to her situation, and J. is an actor who can deliver on that.”

Mr. Lonergan, who had been puttering in the kitchen, wandered back into the living room, still mulling over the question of whether his wife has anything in common with the Waystar Royco general counsel.

“The other thing I was going to say is, J. doesn’t take full command of things but they kind of go the way she wants them to go, sooner or later,” he said. “She’s very strong-willed. At first I would have said there wasn’t any similarity between J. and Gerri, but they both have their eyes on the main point. Both are extremely observant and notice shifts in what’s going on around them. They’re both interested in substance, and neither of them needs to be the center of attention in a room — and nobody is smarter than either of them in a room.”

Ms. Smith-Cameron was beaming. “Thank you,” she said.