The 43-year-old star is playing the lead in the film, which tells the story of the last two years in the life of the Princess of Wales, who died in a car crash in Paris in 1997, reported Daily Star.
Watts has had her long locks trimmed into Diana's trademark wavy bob, and in photos taken on the movie set she can be seen bearing an uncanny resemblance to the royal by adopting her famous doe-eyed facial expression.
The movie is due for release next year.
