The actress recently spoke with ET while promoting her new holiday film, Happiest Season, where she also opened up about preparing to play the late Princess of Wales in an upcoming biopic, Spencer.
"It's a really meditative project," Stewart said. "There are so many perspectives of her and of her story... it's just not a black and white thing and it's a very slippery, really emotionally-packed story for a lot of people."
While parts of Diana's story have been told on film and television several times since her death -- most recently in the fourth season ofThe Crown-- Spencer, from Jackie director Pablo Larraín, will focus on three days of the princess' short life, as she faces the end of her marriage to Prince Charles while spending Christmas at Queen Elizabeth's Sandringham estate in the 1990s. (Based on the timeframe, Prince William and Prince Harry would still be young boys in the film.)
"This is a really kind of poetic, really internalized imagining of maybe the heaviest three days’ time before she does something as gnarly as leaving the royal family," Stewart said. "I'm just sort of reading everything that I can and then forgetting it, because it's kind of a really internalized story."
The actress also appeared on Jimmy Kimmel Live on Tuesday, where she spoke about preparing to play the late royal icon. "It’s hard not to feel protective of her," she said of Diana. "She was so young."
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Stewart, who was just seven years old when the royal icon died in a car accident in Paris in August 1997, admitted she "didn’t know what was going on" at the time, though she remembers seeing the flowers and mourners that gathered outside Buckingham Palace in the days after Diana's death.
"My movie takes place over three days, and it’s this really poetic internal imagining of what that might have felt like rather than giving new information," Stewart added. "We kind of don’t have a mark to hit, we just also love her."
The actress previously admitted that she was finding Diana's accent "intimidating as all hell" in an interview with InStyle. "People know that voice, and it's so, so distinct and particular... I'm working on it now and already have my dialect coach."
She also said she was doing plenty of research for the role, reading multiple biographies and books on the late Princess of Wales.
"It's one of the saddest stories to exist ever, and I don't want to just play Diana — I want to know her implicitly," Stewart said. "I haven't been this excited about playing a part, by the way, in so long."
See more on Diana's on-screen portrayals in the video below.