Naomi Watts has revealed that playing Princess Diana in a film about her romance after her split with Prince Charles has proved her most challenging role yet. The Oscar-nominated actress also admitted that she rejected the offer to play Diana several times but eventually could not bring herself to turn down the role, the Daily Express reported. The British-born star said that the idea of going through this transformation and telling the story of someone who is wildly famous, albeit doing it in a sensitive way, was what drew her to the movie.