Kristen Stewart has said she was so anxious about portraying Princess Diana in the film Spencer that she "couldn't open my mouth for two weeks before we started shooting".
The actress plays the late princess as she spends three days with the Royal Family during Christmas 1991, before she separated from Prince Charles.
Stewart told BBC News: "I had TMJ [her jaw stayed shut] to the point where I was like, completely locked up.
"I was like, 'Huh, I guess I'm really nervous - I was really tripping out until we started."
The film is directed by Chilean-born Pablo Larrain, who previously directed Natalie Portman to a 2017 Oscar nomination for Jackie, about another highly scrutinised female public figure, Jacqueline Kennedy.
The director told Stewart to "relax and trust the process", and rely on her extensive preparation for the role.
The Los Angeles-based actress, who has previously starred in Twilight, said she did not start from a wide base of knowledge.
"I didn't have the most developed or defined relationship with the Royal Family in general," she said. "I didn't grow up following the sort of saga."
But, she added: "Obviously I do live on planet earth, and her impact was so immense and emotional, even for somebody who was seven when she passed away."
Stewart, who has attracted rave reviews for her performance, says she did a lot of research to immerse herself in Diana's life.
"I read everything, I wanted every photo... watched all the interviews that I could get my hands on," she told the BBC's entertainment correspondent Lizo Mzimba.
"I watched The Crown, I watched every iteration of interpretation. I just tried to absorb her in an emotional and general way, and then trust the process, and expect her to show up."
The weight of responsibility in playing such an emotive, hugely known public figure meant Stewart had to anchor the portrayal by basing it on her own feelings, as there are so many opinions about the princess.
"I felt I wanted to protect her," she said. "I had to just not focus on other people's idea of her, and really focus on my own. And that in itself was just so distinct and specific to me."
Stewart says she was keen not to let Diana's memory down, explaining: "I think to do her justice is to allow her to be impulsive. Anything I watched her in, whether it was an interview, or even in a still photograph, it always feels unpredictable. Like you don't know what's going to happen.
"And it's because she has this vulnerability and this raw emotion that she cannot conceal. There's no way to do a perfect impression of that.
"You have to feel it, and it has to be yours. So I think I just had to relax."
The film, set over three intense days in the royal country residence Sandringham in Norfolk, is described by Stewart as a "tumultuous tone poem that also felt really exuberant and wonderful and joyous".
It shows the princess in great emotional distress at times, but actress said it still has exuberance because "in an odd way... it is about her fight to embody her life and and have some agency, and get away from the weight that the Royal Family obviously brought to her life".
Stewart admits that the film is an "imagining of what it might have felt like, over the course of this decision to leave the family", but adds she does not feel it is a "betrayal" of the princess because it is "a poetic interpretation of all things that we absolutely know".
The actress said her feelings for the princess grew as she played, her, explaining: "I felt such love for her and still do... I wish I could ask her if she thinks I'm doing a good job."
The film does not shy away from the princess's eating disorder and struggles with her mental health.
"The movie is very surreal. I think that when you go through extreme trauma in life, there are times where you feel crazy. I never felt the implication of Diana, losing it," Stewart said.
"There are there are times where you are at odds with communication and therefore feel because of this muzzled energy, a bit locked in your own head.
"And she spoke about that. I mean, that was something that she talked about all the time. And so I felt to be honest, it felt it felt truer than the truth."
Stewart has already been talked about as a contender for next year's Oscars - something she has embraced.
"It was really cool. I've never had that," she said.
"We make movies to have the most open and large-scale conversations that we can have, as a culture and like a society. It's nice to be just for a second a part of that - it's fun."