Watts will play the title role in Diana, the biopic directed by Oliver Hirschbiegel and currently shooting in London.
© Laurie Sparham
She spoke to News.com.au about her initial trepidation in taking on the role, arguing that playing "one of the most famous, if not the most famous, women of [our] time" comes with "an awful lot of pressure".
She continued: "I was dreading people saying, 'She looks nothing like Diana', and, 'Why her and not somebody else?' But I couldn't not do it."
She added that the film has led to her "dreaming" about the late royal, adding: "I felt like I was spending time with her. One particular time, I felt that permission [to play the role] was granted."
Watts's transformation into Diana for the film is reported to have involved six weeks of training with a dialogue coach, repeated viewings of the famous Martin Bashir interview, four wigs and a prosthetic nose.
Watts added in the interview that she is conscious about how Diana's sons Princes William and Harry will react to the film, saying: "We're not trying to destroy any memory. We're trying to show her in the best possible light as a human being, though, full of different layers.
"It's a story that should be told. A piece of history. But it's tricky because they are here, the boys. I think about that a lot."
Director Hirschbiegel has said that the similarity between Watts and the real Princess is uncanny, saying: "It's scary. Sometimes it's as if you're watching the ghost of Diana."
