The British-born actress, who grew up in Australia, stars in Oliver Hirschbiegel's upcoming drama "Diana", which tells the story of the last two years of her life before she died in a car crash in Paris in 1997, reported Daily Express.
"I am dreading the reaction amongst the pubic to my playing her. You know that people will complain that you don't look enough like her, that you're not tall enough, all those things.
"It's very difficult to play someone who had such a deep impact and left such a lasting memory on the public. Our memory of her is still very fresh and you have to be respectful of that," she said.
Watts, 43, said she will always remember the day Diana passed away.
"I was shooting a movie in Canada when she died. I was in a room full of people watching the news about Diana's death and we were all crying and deeply upset. It's one of those events you never forget."