When Margaret Qualley auditioned for the part of Unknown Manson Girl in Once Upon a Time…in Hollywood, Quentin Tarantino’s ode to Los Angeles in 1969, she was asked to sing and read some lines, and thought she had done pretty well. “But afterward, my agent called and said it was a ‘no go,’ ” Qualley told me over tea at Loews Regency hotel in New York nearly two years later. Even though it was a steamy summer evening, she was wearing a giant red hooded sweatshirt, and her long, dark, wavy hair was encased in fleece. “I never wear anything revealing in real life,” she explained. “Even if it’s scorching, I don’t like to show any skin. I save that for movies.”

Qualley had worn a period-appropriate blouse and jeans to her audition—an outfit she was second-guessing after she was turned down, until her agent called to say that Tarantino had rewatched all the tapes and changed his mind. Could she come back and read with Brad Pitt to see if they had chemistry? “I was like, yes!” Qualley said, jumping up and down in her seat. “I was in Panama visiting my dad. It was all beach huts and no cell phone service, and my agent was frantically trying to reach me. I flew to L.A., met Quentin and Brad, and got the part!”

Qualley wears a Viktor & Rolf Couture vest, top, and skirt; stylist’s own head scarves. Beauty note: L’Oréal Paris Infallible Fresh Wear Foundation in Rose Ivory, Colour Riche Plump and Shine Lipstick in Pitaya Plump.

Photograph by Inez and Vinoodh; Styled by Sara Moonves. Hair by Jimmy Paul at Susan Price NYC; Makeup by Sil Bruinsma for Chanel at the Wall Group. Produced by VLM Productions; Executive Producer: Stephanie Bargas; Producer: Tucker Birbilis; Studio Manager: Marc Kroop; Production Coordinator: Eva Harte; Lighting Director: Jodokus Driessen; Digital Technician: Brian Anderson; Photography Assistant: Joe Hume; Fashion Assistant: Allia Alliata di Montereale.

In Once Upon a Time…in Hollywood, Qualley plays Pussycat, the hippie temptress who lures Pitt’s character to the Spahn Movie Ranch, where the Manson family lived. “I had to lose a lot of wonderful scenes in the film,” Tarantino told me, “but I never cut or trimmed any of Margaret’s. She was the only person that I didn’t edit down.” In fact, he wrote more lines for her. “Quentin said he couldn’t sleep and came in the next day with handwritten pages,” Qualley recalled. “We had to decipher his writing. It was so thrilling!” She followed Tarantino around with a notebook, writing down his thoughts or anything she found interesting on set. “At one point, Lena Dunham [another Manson girl in the film] asked Brad, ‘Do you ever play ­losers?’ And Brad said, ‘I do. But I always make them win.’ ” Qualley smiled. “Words to live by.” During the shoot, Qualley wasn’t bothered that Tarantino wouldn’t let the Manson girls shave their legs or their armpits, but she was dismayed by his fascination with feet. “Quentin wanted my weird-looking feet up on Brad’s fancy car’s dashboard,” she said, still cringing at the memory. “I kept trying to hide them, but he insisted.”

Viktor & Rolf Couture vest, top, and skirt; stylist’s own boots (throughout).

Photograph by Inez and Vinoodh; Styled by Sara Moonves.

Ronald van der Kemp dress.

Photograph by Inez and Vinoodh; Styled by Sara Moonves.

Qualley, who is now 24, trained to be a ballet dancer since she was a child, and her feet have paid the price. She grew up in Asheville, North Carolina, with her mother, the actress Andie MacDowell, and her two older siblings: Rainey, who is a singer, and Justin, who works on a ranch. Her father, Paul Qualley, was a model when he met her mom on a commercial shoot for the Gap (“I always say I’m the O.G. Gap kid”); they divorced when Qualley was five. She left home at 14 to attend a boarding school for dance and was about to start an apprenticeship with the North Carolina Dance Theatre when she decided to quit dancing altogether.

Valentino Haute Couture dress.

Photograph by Inez and Vinoodh; Styled by Sara Moonves.

Chanel Haute Couture dress.

Photograph by Inez and Vinoodh; Styled by Sara Moonves.

“When I was 12 or 13, I couldn’t imagine any other life than ballet,” Qualley said. “But at some point I started to hate it. All of a sudden, I fell in love with acting.” Qualley took some classes at the Royal Academy of Dramatic Art in London and almost immediately was cast in the series The Leftovers, where she played a survivor of a mysterious, Rapture-like phenomenon in which some people simply disappear. At the beginning of her career, Qualley’s unique beauty, which can seem both otherworldly and completely natural, lent itself to roles with religious overtones: She also played an aspiring nun in Novitiate, and an end-of-the-world beekeeper in the dystopian drama Io. Perhaps it’s because she started dancing again at 22—most notably in a short, joyous film directed by Spike Jonze for the fragrance Kenzo World—but, recently, Qualley has been taking on looser, less spiritual characters. In July, she was nominated for an Emmy Award for her work in Fosse/Verdon, an eight-part series about the complex collaboration and marriage between the director-choreographer Bob Fosse and the dancer Gwen Verdon.

Viktor & Rolf Couture dress.

Photograph by Inez and Vinoodh; Styled by Sara Moonves.

Valentino Haute Couture dress and hat.

Photograph by Inez and Vinoodh; Styled by Sara Moonves.

In that series, Qualley plays Ann Reinking, Fosse’s lover and muse, and is quietly transfixing in the part; while Fosse and Verdon rage and manipulate, Reinking is the calm center of the drama. And Qualley’s dancing is stunning: She nails the complicated Fosse moves and style. “Ann Reinking lives in Arizona now,” Qualley said, as she poured some mint tea. “She is 69 and has a lot of personality. Every time I went crazy on that show and felt like I was out on a ledge and ready to jump, she pulled me back in.” From repeated viewings as a kid, Qualley had already memorized all of Reinking’s numbers in All That Jazz, but she became obsessed with studying every dance film she could find. “I watched Flashdance over and over,” Qualley said. “And when I got bored, I figured out the dances in music videos. I can do every step of ‘Thriller,’ and I love the end of Björk’s ‘It’s Oh So Quiet.’ And don’t get me started on Britney Spears’s ‘Oops!…I Did It Again’! That’s, like, wow!”

Polaroids taken by Margaret Qualley while filming Once Upon a Time…in Hollywood.

Photograph by Margaret Qualley.

Polaroids taken by Margaret Qualley while filming Once Upon a Time…in Hollywood.

Photograph by Margaret Qualley.

A handwritten list by Quentin Tarantino of some of the actresses in his latest feature.

Photograph by Inez and Vinoodh; Styled by Sara Moonves.

Qualley had to leave—she was meeting a friend for dinner at a sushi restaurant. She had had a busy day, with a fitting for the custom beaded Prada dress she would be wearing to the Once Upon a Time…in ­Hollywood premiere in L.A., a tour of a new apartment she was considering renting, and a visit to the Metropolitan Museum of Art. “I’ve still got the sticker on my arm,” she said. Qualley has a few upcoming films, including Seberg, about the actress Jean Seberg’s involvement with the civil rights movement in the late 1960s. But it’s her role as Pussycat in Tarantino’s film that best captures her radiant-child exuberance. “I did like being that girl,” Qualley said as we left the hotel. “I was even a little sad to shave my legs after the shoot was over.”

Chanel Haute Couture dress.

Photograph by Inez and Vinoodh; Styled by Sara Moonves. Hair by Jimmy Paul at Susan Price NYC; Makeup by Sil Bruinsma for Chanel at the Wall Group. Produced by VLM Productions; Executive Producer: Stephanie Bargas; Producer: Tucker Birbilis; Studio Manager: Marc Kroop; Production Coordinator: Eva Harte; Lighting Director: Jodokus Driessen; Digital Technician: Brian Anderson; Photography Assistant: Joe Hume; Fashion Assistant: Allia Alliata di Montereale.