Stewart said she embraces the pressure that comes with portraying a beloved icon.
"It makes me stand at attention in the best way," the star said.
"I feel sort of the same way about her, and it happened really quickly," she added. "I didn't grow up with her in the same way. I was really young when she passed away. I remember the flowers ... I was really young, didn't really know what was going on."
Stewart said she's come to feel protective of Diana, who died in a car crash at age 36 in August 1997.
"It's hard not to feel protective of her. She was so young," she said. "Everyone's perspective is different, and there's no way to get anything right because what is fact in relation to personal experience?"
"My movie takes place over three days," Stewart said. "It's this really poetic internal imagining of what that might have felt like, rather than giving new information. So we kind of don't have a mark to hit; we just also love her."
Stewart confirmed she's preparing for the role by watching videos of Diana and listening recordings of Diana's voice.
"I fall asleep with that in my head," she said.
Stewart also stars in the new film Happiest Season, which premieres Wednesday on Hulu. The movie is a holiday romantic comedy starring Stewart as Abby, a young woman planning to propose to her girlfriend (Mackenzie Davis) during the holidays.