BEFORE her untimely death on August 31, 1997, Princess Diana was the world's most famous woman, and for that reason it was going to take a brave actress to portray her on screen. Naomi Watts stepped up to the challenge but, understandably, it wasn't a decision she took lightly.
"I was very nervous about taking on the role," says the 44-year-old actress.
"It's the biggest challenge of my career thus far. Taking possession of a character that everyone feels they know so well, and therefore belongs to them, was difficult and scary. There's more information available on this woman than any person we have known in our lifetime.
"So that's a huge scary challenge, and I couldn't say yes very easily. But those reasons also became the intrigue, and the very fact that I was afraid of them made me turn around and take the role."
Watts, who has twice been nominated for Best Actress at the Oscars, in 2004 for 21 Grams and 2013 for The Impossible, understands that film fans will question her resemblance to the late "People's Princess", and journalists will pore over her performance in the film, which is based on the last two years of Diana's life and her relationship with heart surgeon Dr Hasnat Khan, played by Lost actor Naveen Andrews.
But the one person the British-born actress craved approval from was her own mother.
"Diana's voice was something we all remember very well," she says. "There was a lot to get right, and even my mother said to me, 'Oh God, I don't know if you'll be able to get that voice, Naomi!'
"Mum knew how tough that accent would be to get. What made it tougher was that Diana's wasn't just pure stiff upper lip, she had a very breathy modern sound to her voice. It wasn't as posh as it could have been."
The actress got the thumbs-up she was hoping for after her mother watched a preview.
"Mum and her partner, who is quite posh himself, were impressed with the accent after all," she says, laughing.
Not everyone has been so kind. While Watts mastered Diana's dulcet tones and mannerisms, the film, which includes several key moments from Diana's final years including her BBC Panorama interview with Martin Bashir, landmine charity campaign and holiday with Dodi Fayed shortly before the Paris car crash that killed them, has received decidedly mixed reviews from critics.
And, according to recent reports in the press, Watts is feeling the pressure.
Although she doesn't seem entirely at ease when we meet, she is nevertheless friendly and personable, and jests with director Oliver Hirschbiegel, who is being interviewed alongside her.
Watts recently joked that she would leave the country to escape the spotlight when the movie was released, but the comment provoked more attention than she had anticipated.
"Anything you say ends up being interesting when it's about Diana, because we're so endlessly fascinated by her," she says.
"I think it's testament to who she was, and what she achieved, and we're struggling with why she's not here."
If Watts, whose career began with TV work before she went on to land roles in films like King Kong, I Heart Huckabees and The Painted Veil, does fancy escaping the hubbub, there is one place she could retreat to – the north-west coast of Wales.
The actress, who moved to Australia in her teens but now lives in the US with her actor husband Liev Schreiber and their two sons, Alexander and Samuel, spent a chunk of her childhood in Anglesey, where her grandparents lived.
Prince William, Diana's eldest son, made a home for himself and his wife Kate on the island while he served as an RAF search and rescue pilot.
So has Watts been back there?
"I haven't quite found time to get back to Wales, but I have strong memories of my time in Anglesey," she says.
"My grandfather, Hugh Roberts, was a great figure in my life because my father died when I was very young, so I remember Anglesey with great fondness."