The Hollywood actress is set to star in Spencer, which will tell the story of a weekend in the early 1990s when Diana decided her marriage to the Prince of Wales was failing.
Stewart, 30, revealed production is scheduled to begin in mid-January and in the meantime she is doing extensive research so she knows the princess “implicitly”.
She told InStyle: “The accent is intimidating as all hell because people know that voice, and it’s so, so distinct and particular. I’m working on it now and already have my dialect coach.
“In terms of research, I’ve gotten through two and a half biographies, and I’m finishing all the material before I actually go make the movie.
“It’s one of the saddest stories to exist ever, and I don’t want to just play Diana – I want to know her implicitly. I haven’t been this excited about playing a part, by the way, in so long.”
Twilight and Charlie’s Angels star Stewart also recalled the pressure she felt to publicly discuss her sexuality.
“The first time I ever dated a girl, I was immediately being asked if I was a lesbian,” she said.
“And it’s like, ‘God, I’m 21 years old.’ I felt like maybe there were things that have hurt people I’ve been with. Not because I felt ashamed of being openly gay but because I didn’t like giving myself to the public, in a way.”
Stewart, who previously dated British actor Robert Pattinson, added: “Even in my previous relationships, which were straight, we did everything we could to not be photographed doing things – things that would become not ours.
“So I think the added pressure of representing a group of people, of representing queerness, wasn’t something I understood then.”
Spencer will be directed by Pablo Larrain and the script has been written by Peaky Blinders creator Steven Knight.
