The actress was cast as the late British princess in Oliver Hirschbiegel's upcoming biopic last year (12) but her immediate concerns were the feelings of Diana's sons, Princes William and Harry.
Watts' father, Peter, died of an apparent drug overdose when she was just seven years old and she was determined to make sure her part in the movie does not upset the royal brothers, who were aged 15 and 12 when Diana died in a car crash in Paris, France in 1997.
She tells British magazine Event, "I've thought about her sons, of course, and it's haunting that they lost their mother because I know what it means to lose a parent so young.
"I'm always thinking about those boys - I can't imagine growing up without a mother. Of course it was in my mind when I made the decision to do the film; how would they feel? I would hate to upset them. I hope they feel good about it. It's a piece of history that we are all interested in and at some point the story had to be told."