Emma Eastwood had just changed out of Barbie’s tight pink jumpsuit at the Warner Brothers studio in London when she learned she had landed her most high-profile role yet: Barbie’s body double.

The news came via text from one of the casting directors of Greta Gerwig’s new movie, just hours after she auditioned for the role. She had set her expectations low after they twice rescinded offers for her to be an extra, she said. “I was fully convinced that I kept jinxing myself,” Ms. Eastwood added.

The 26-year-old actress, who grew up in San Francisco and now lives in London, had mostly worked on commercials, music videos and short films. Her role in “Barbie” marked her first time as a body double.

Any anxieties were assuaged her first day on set. She woke up before sunrise, and by 6 a.m., she was on a minibus that transported her to the studio.

“My first couple of days, I got a bit of the star treatment,” she recalled, pointing out that she was one of the few people on set at the time. “I felt really spoiled,” she said, adding that she was surprised to find out she had her own trailer.

In the casting world, body doubles are hired to fill in for actors who aren’t able or willing to film certain scenes. In other cases, stand-ins can help the production team save both time and money. “They’re not going to have Margot Robbie sit on a set while they’re doing lighting tests,” said Liz Lewis, a casting director who did not work on “Barbie.” “They’re going to pick somebody who has the same size, height and coloring.”

Ms. Eastwood was initially hired as the hand double of Ms. Robbie, who plays the titular character, but ended up taking on more robust scenes where her whole body, aside from her face, was featured in the shot. “They didn’t give me any details of what we would be doing,” she said. “There were a couple times they said the whole cast would be there and it would be an important day, but they never actually gave me any details of what we would be doing until I was on set.”

Ms. Eastwood’s main goal was to befriend Ms. Robbie in hopes of working as her body double on another film. The pair, however, didn’t overlap often. While Ms. Eastwood filmed her scenes on one stage, Ms. Robbie was often filming hers on another.

Ms. Eastwood felt confident about her work: She said people on set often mistook her for Ms. Robbie, including the actress herself. While looking back at the footage, Ms. Robbie told her that at some points she thought she was watching herself.

And even if their schedules did not align, their measurements did. Every costume Ms. Eastwood wore was one that Ms. Robbie had as well. “They didn’t make anything for me specifically,” she said.

Every morning before filming, Ms. Eastwood would spend about an hour getting her hair and makeup done. The makeup artist covered her in a bronze foundation that more closely resembled Barbie’s sun-kissed veneer. But even after several layers were applied, the head of the makeup team would often approach the artist doing Ms. Eastwood’s body paint to let her know the shade wasn’t dark enough.

To avoid staining the costumes, she wore her own clothes until it was time to shoot the scene. From that point onward, Ms. Eastwood often had no idea what would come next. Without any context for the plot of the movie, she was open to doing anything that was asked of her.

One of the scenes Ms. Eastwood filmed several times was featured in the trailer — a shot of her walking up a set of stairs after she was directed to act like her feet hurt. When the trailer was released, she was excited to see that it was included.

In another scene, Ms. Eastwood had to lie on the ground facedown for an hour. Her makeup smeared as she kept one side of her face on the floor. “When I got up, I literally felt drunk,” she said. “I have no idea what that scene was about.”

And that sentiment applies more broadly to the entire experience. “I was on this for two weeks, and I barely know what the movie is about,” she said. “They did a very good job of keeping the plot hidden.” She will not be able to see the movie until it is released to the public this summer.