• Kristen Stewart could not open her mouth for two weeks before she started filming "Spencer"
  • Stewart said she was so nervous playing Princess Diana that she was "tripping out" until shooting began
  • The "Twilight" star read all the material and watched all the interviews of Princess Diana to prepare for the role

Kristen Stewart is speaking up about her fears over playing Princess Diana.

The "Twilight" actress revealed in an interview with BBC News that she was so anxious about portraying the late princess in "Spencer" that she "couldn't open my mouth for two weeks before we started shooting."

"I had TMJ [temporomandibular joint disorder, meaning that her jaw stayed shut] to the point where I was like, completely locked up," Stewart admitted. "I was like, 'Huh, I guess I'm really nervous - I was really tripping out until we started."

The temporomandibular joint acts like a sliding hinge, connecting the jawbone to the skull. TMJ disorders can cause pain in your jaw joint and in the muscles that control jaw movement, according to Mayo Clinic. The exact cause of a TMJ disorder is often difficult to determine.

In "Spencer," Stewart plays Princess Diana as she spends three days with the royal family during Christmas 1991, before she separated from Prince Charles. The film is helmed by Pablo Larrain, who previously directed Natalie Portman to a 2017 Oscar nomination for "Jackie," which centers on another highly scrutinized female public figure, Jacqueline Kennedy.

Larrain told Stewart to "relax and trust the process." She was also advised to rely on her extensive preparation for the role.

Stewart admitted that she hasn't really followed the royal family's affairs. But she was aware of Princess Diana's impact, so she felt the pressure of portraying the late Princess of Wales on the big screen.

"I didn't have the most developed or defined relationship with the royal family in general. I didn't grow up following the sort of saga," the Los Angeles-based actress explained. 

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."

To prepare for the role, Stewart read every material and watched all the interviews of Princess Diana that could get her hands on. She also watched Netflix's "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," she shared.

"Spencer" premiered at the Venice Film Festival on Sept. 3, just four days after Princess Diana's 24th death anniversary. Stewart's portrayal of the late royal generated buzz, and most were positive. Some film critics even predicted that Stewart could score an Oscar nod for "Spencer."

"'Spencer' is [a] haunting & deeply complex character study. A phenomenal film, easily one of the year's best. You will not see a better performance this year than Kristen Stewart as Princess Diana. Brilliant filmmaking, storytelling, & score. Kristen is a lock for Oscars," Scott Menzel tweeted.

Film critic Scott Mantz also praised Stewart's performance, saying he was "completely floored" by her "immersive commitment" for the film, which was "beyond incredible."

"Film is an engrossing mood piece & a portrait of #PrincessDiana as a suffocated prisoner looking to make a jailbreak," Mantz added.

"Spencer" will be released on Nov. 5.

There was immediate awards buzz around Kristen Stewart for her role as Princess Diana in "Spencer There was immediate awards buzz around Kristen Stewart for her role as Princess Diana in "Spencer" Photo: AFP / Filippo MONTEFORTE