The 31-year-old actress recently spoke to Entertainment Tonight about her role as Diana in the upcoming film, explaining why is felt strange to wear the wedding gown in particular.
"When we put that dress on, it is embedded in a montage that kind of flashed her life before you. Even though the movie takes place over a three-day period and even though I play her as a speculatively 29-, 30-year-old person, we still wanted to give a taste of the scope of her life," Stewart told the outlet. "Kind of touch on every part, even if it was in kind of lyrical dance. And the wedding dress is embedded in that montage and putting that on was a spooky day."
Though Stewart said there won't be a scene of the wedding itself in the movie, director Pablo Larraín helped her imagine how Diana might have been feeling just before her nuptials to Prince Charles.
Related video: Kristen Stewart wore an ab-baring bra top to Spencer premiere
"We didn't have to do the wedding scene but I still had one walk, walking outside of this castle," she told ET. "I was like, 'Wait, but this is so particular.' This actually does land on a very precise day of her life and [Larraín] was like, 'Take everything you learned about that day, all of your emotional responses that you have accumulated, put them all right here. What does it feel like to believe in an ideal that then becomes very, very apparently farcical? It's kind of just like, let your heart break in a moment.' "
"I was like, 'Okay got it. Let's go,' " Stewart said.
Never miss a story — sign up for PEOPLE's free daily newsletter to stay up-to-date on the best of what PEOPLE has to offer, from juicy celebrity news to compelling human interest stories.
"Kristen Stewart doesn't just do an impersonation (though on the level of impersonation she's superb). She transforms; she changes her aspect, her rhythm, her karma," Owen Gleiberman wrote for Variety. "Mostly, though, what we see in Stewart's Diana is a woman of natural-born elegance, with a luminosity that pours out of her, except that part of her is now driven to crush that radiance, because her life has become a wreck."
The film is set over the course of a weekend in December 1991 when an anguished Princess of Wales joins the royal family for Christmas at the Sandringham Estate and decides to leave her fractured marriage.
"Spencer is a dive inside an emotional imagining of who Diana was at a pivotal turning point in her life," Stewart previously said of the part. "It is a physical assertion of the sum of her parts, which starts with her given name: Spencer. It is a harrowing effort for her to return to herself, as Diana strives to hold onto what the name Spencer means to her."
Spencer hits theaters November 5.