Kristen Stewart "couldn't open [her] mouth for two weeks" before she started shooting 'Spencer'.
The 31-year-old actress portrays Princess Diana in the new biographical drama film, but she admits it was a nerve-wracking experience, and the anxiousness actually led Kristen to develop a pain in her jaw.
Kristen - who plays the late Princess as she spends three days with the Royal Family over Christmas in 1991 - told 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 Hollywood star revealed she didn't actually know a huge amount about the Royal Family before she accepted the role.
She explained: "I didn't have the most developed or defined relationship with the Royal Family in generally. I didn't grow up following the sort of saga.
"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."
However, Kristen did a lot of research before she started shooting the new movie.
The actress even took to watching 'The Crown' as part of her preparations for the role.
She said: "I read everything, I wanted every photo ... watched all the interviews that I could get my hands on.
"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."
Kristen also felt a need to "protect" the late Princess.
She added: "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."