Princess Diana is one of the world's most beloved women, so it's not surprising that Naomi Watts is "absolutely terrified" of playing her.
Naomi, 43, has signed on to play the late royal in new film Caught in Flight, which focuses on the last two years of Diana's life.
The Australian actress was thrilled when she was offered the role, but says she was tempted to turn it down because she was so worried about doing the part justice.
"She is a part of our history, an incredible woman and fascinating but a tragic ending," Naomi told the UK's Daily Mail.
"The film-makers came to me and it's a good script and it's very scary and I kind of wanted to say no but I couldn't."
Naomi is glad she decided to accept the role, but says she is still worried about what the finished movie will be like.
She is spending all her spare time researching Diana — who died in a Paris car crash in 1997 — and hopes the film will uncover something new and interesting in the well-known story.
"Now I'm doing all the research and there is so much to read, I just hope we make a good film and people can learn something new about her that's interesting," Naomi said.
"There are a lot of people that knew her and a lot of books to read and so much information and one story is different to the next so it's hard to gauge what was the truth.
"It's going to be a tough one, it's a big beast to take on, I just try to do my best."
Caught In Flight will be directed by Oscar nominee Oliver Hirschbiegel and filming is scheduled to begin in the UK next year.
It won't be the only Diana movie in cinemas in 2013 — two other biopic projects are reportedly in the works.
Closely Guarded Secret will be based on the book of the same name written by Diana's bodyguard Ken Wharfe.
It will chart Diana's life in the lead-up to her much-publicised divorce from Prince Charles.
Another film, simply titled Diana is also planned, but it is not known what parts of the princess' life it will cover.
Naomi was born in Britain and moved to Australia when she was 14 years old. She now lives in New York with her husband Liev Schreiber and their sons Alexander, four, and Samuel, three.