The first-time Oscar nominee, 31, tells PEOPLE ahead of the March 27 ceremony that "Diana Spencer made people feel good," and that she played the late royal "at a time that was hard and tumultuous and sad, but she also has this unstoppable light."
"And to even attempt to try and touch that, I learned a lot, but specifically, I felt like I was allowed to grow as a person," Stewart explains. "Even if it was in my imagination and I was completely convincing myself that suddenly I had this superpower that she had, which was to make people feel good."
"While we made the movie, I felt like I had that, and it was a really beautiful feeling," she adds.
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Shoebox Films Kristen Stewart as Princess Diana in Spencer (2021)
Stewart says she loves what the movie is about, specifically, "If you can't show yourself and you have a cut-off portal to your life."
"Diana, especially in the time that we deal with her, she's very muzzled," the actress explains. "The opportunity to have this exchange, it is not lost on me any second of any day how cool that is."
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She also shouts out the "integrity" of her costumes: "Sometimes, wardrobe can feel like wardrobe — you know that every pocket doesn't necessarily work, or if something's built ... you feel in costume."
"I never felt like I was in costume, and [she's] the most extreme character I've ever played," Stewart says. "And she expressed herself in the things she put on her body. Sometimes it was armor, sometimes it was to ruffle people's feathers."
OSCAR NIGHT! Watch live on ABC March 27 at 8 p.m. ET. And before the awards, tune in to People & Entertainment Weekly Red Carpet Live, starting at 5:30 p.m. ET on People.com, EW.com and PeopleTV.
Tim Graham Photo Library via Getty Images Kristen Stewart (L); Princess Diana
Stewart says she is "thrilled" to land her first-ever Academy Award nomination for her work in Spencer, and that the feeling of being recognized is "all the things you would think it would be."
"It's incredibly surreal — I never imagined myself in this arena," she admits. "I would make movies for free."
The 2022 Oscars will be held Sunday, March 27, at Hollywood's Dolby Theatre and televised live on ABC at 8 p.m. ET.