Watts, 44, said she rejected the offer to play Diana several times but eventually could not bring herself to turn down the role, reported Daily Express.
"I was nervous going into it and tried my best to say no a couple of times but I wasn't at peace with that.
"I guess I wanted to do it for all the reasons I was against it. The idea of doing this transformation and telling the story of someone who is wildly famous, albeit doing it in a sensitive way. She is a fascinating woman with a fascinating story that obviously ended in such tragedy," she said.