In a curious way, the Oscars are the culture’s yearbook. For nearly a century, the class heroes have annually paraded in front of the cameras for us to measure and judge, giving us an unexpected snapshot of shifts in our mores and expectations. And one of the delights of paging through Dijanna Mulhearn’s “Red Carpet Oscars,” an impressively researched and organized new book, with a foreword by Cate Blanchett, is how clearly it outlines both the evolution of the Academy Awards through clothing and an immutable truth about dress.

“No matter how arbitrary they seem, clothing choices intrinsically reflect something about the wearer,” Ms. Mulhearn writes. Just as inevitably, they mirror the society in which we live.

From her home in Sydney, Australia, where she was packing to attend the Oscars week festivities (although not the red carpet, alas) as Ms. Blanchett’s guest, Ms. Mulhearn spoke to The New York Times last week about her book that was initially conceived during lockdown as a scholarly work on semiotics, but later transformed into something entirely different — and much juicier.

“It was a densely researched project that almost turned into a textbook on clothing,” Ms. Mulhearn said. “But my agenda was to reach the masses, and so I had to pivot.” What is the opposite of academia? Entertainment, according to Ms. Mulhearn, who added: “And what is the Oscars but entertainment’s pinnacle moment?”

Guy Trebay: Something you point out in the book struck me as particularly worth remembering about movie people: Even in the democratized age of social media, they inhabit a separate plane. Yes, we’ve all seen paparazzi shots of them mooching around with their sweats and to-go cups from the Coffee Bean. But on this one night, they revert to their roles as demigods.

Dijanna Mulhearn: Stars are still as close as most of us are likely to come to royalty. And since, generally speaking, the people on the red carpet came from the same places as the rest of us, we can relate to them. You can’t necessarily imagine yourself in a movie, but you can imagine yourself in the dress and feel like you’ve got a little bit of that magic.

GT: Yet, given that the Oscars began in 1929, it took Hollywood a minute to figure that out.

DM: The Oscars were initially created and controlled by the studios as a publicity tool. At first, no one knew what was going on.

GT: They did know who the winners were, though, since they’d been announced months before.

DM: Yes, but on the night of the first Oscars dinner, Janet Gaynor, who won for three movies, came straight from a full day of shooting. She rocked up at the dinner in whatever she had.

GT: Hard to believe it was a blouse with a Peter Pan collar and a knit skirt picked up at a thrift shop, as you write.

DM: But look at the subsequent years. Mary Pickford was on the committee in 1930 and had ordered her dress for the Oscars, which she won, before the movie was even finished. The following year Norma Shearer collected her Oscar in a dress that was modified from something worn in the movie she’d won for. Pretty quickly people were thinking about associating their red carpet clothes with what was happening onscreen, in their careers and with the messages they wanted to convey.

GT: They were shaping image or, I guess, branding.

DM: I’m very interested in clothing as communication, the kinds of things we say before we even open our mouths. Probably everyone is aware of the language of clothing, of dress as a language. What’s less well known is how effectively those who are more fluent in it — particularly in an image-based world — get ahead.

GT: Zendaya comes immediately to mind.

DM: Zendaya is a super-interesting case. Red carpet dressing is a very thoughtful and strategic process. It is not just a matter of “I’m a beautiful woman with a fabulous body, put something on me.” The messaging is complex. The most successful celebrities, the ones likely to have any consistent career length, approach it with a great level of detail.

Last year, Zendaya wore a skirt and cropped shirt by Valentino. It was beautiful, but it also dipped into historical tropes. It was a nod to Sharon Stone’s famous Oscars moment in a Vera Wang skirt and a collared shirt that belonged to her husband. And, in the minds of an older generation, the look ties Zendaya to Stone and her success and that implicitly creates a longevity through the visual association.

GT: In a sense, it’s a history of glamorous eye candy, yet it’s also a record of careers conducted like military campaigns.

DM: Clothing can be so much more than simple adornment. All the choices we make are reflections in some way of who we are. So many things are being said on the red carpet, with people trying to layer meaning into that one moment.

GT: For example?

DM: Look at Bette Davis. Recognizing that studios are controlling everything, hating the scripts she’s being offered, insisting she wants to do grittier roles, she accepts her best actress award in 1936 in a dress made by Orry-Kelly that looks like a maid’s outfit. It was a clear statement against the studio that she felt was treating her like the hired help.

Three years later, when she wins again for “Jezebel,” things have changed. She’s in the throes of making “The Private Lives of Elizabeth and Essex,” and her costume picks up on all the regal cues.

GT: I’ll say. She’s wearing a zany Collette dress with a billowing skirt and an egret feather collar that resembles some fierce form of avian dominance display.

DM: Davis was making a statement. She was striding in and saying, “I am the queen of Hollywood.” And she remained queen of Hollywood for quite some time. She used her clothes to communicate her position. That’s what the red carpet is for.

GT: Perhaps we think a little bit less about this now that so many celebrities are tied to designers and big fashion houses that — I don’t think we’re betraying state secrets here — not only provide the clothes they wear on the red carpet, but reward them lavishly when they do.

DM: Actors are in a tough position. There are so many pressures on them from so many levels. There are a lot of stakeholders in a successful actor’s career. If you look back to the ’90s, when Armani began making sophisticated red carpet clothing for actresses who didn’t want to be defined by frilly clothing, you begin to see this shift.

GT: No one capitalized on this commercial symbiosis sooner or better than Armani.

DM: No, and then afterward, people began flocking to professional stylists to help them so they wouldn’t be vilified. They were afraid of being called out by Joan Rivers. They were terrified of looking shallow. So they began to wear beautiful though not particularly interesting garments. Despite all the professionals, I don’t think a lot of them understand the history. Producers are looking at the kind of attention you’re getting with the public in order to decide whether to cast you in their next film.

GT: And yet, in recent years, there is more thoughtfulness, with celebrities like Billy Porter testing gender norms and stars like Cate Blanchett recycling red carpet dresses and jewelry.

DM: Hollywood has always tended to reflect social shifts, whether consciously or not.

GT: I immediately think of Julie Christie in a miniskirt bought off the rack, Joanne Woodward sewing her own Oscars dress, Barbra Streisand wearing a transparent Scaasi pantsuit, Sonny Bono in a beaded Nehru collar jacket and Cher in virtually anything.

DM: Or think of Audrey Hepburn and Grace Kelly at the 1956 Oscars, when the archetypes were shifting. We were exiting the period of the sex symbol with a voluptuous figure and a troubled past and supplanting it with princess archetypes. That princess figure had disappeared during wartime when people had so many more serious issues to confront. Now suddenly here was Kelly in yard and yards of fabric as the girl next door embodied in a highly sophisticated way, and Hepburn in an uncomplicated style ….

GT: By, ahem, the French couturier Hubert de Givenchy.

DM: Yes, but in a look that somehow seemed possible to emulate. In society now there’s a lot of awareness of consumerism and waste. There’s real thought being given to the drive to have something new every five minutes, like some kind of addict, always getting something more from the next garment. That’s why Cate Blanchett, who wrote the book’s foreword, stands out. She has a genuine hand in what she wears. She’s not an actress who lets someone choose something for her.

GT: Still, whoever is behind the scenes pulling the levers and however much we all play Monday morning fashion quarterbacks, the Oscars remain the one night we all succumb to the spectacle of clothing.

DM: Oh, absolutely. The magic is real.

This interview has been edited for length and clarity.