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.”