• Share
  • Tweet
  • Share
Naomi Watts

Dave M. Benett/WireImage

Being a royal is hard work—just ask Naomi Watts.

The actress is only playing Princess Diana in the film Diana and yet she spent three intense months just prepping her body for the role.

The star's trainer Simone De La Rue told Heath & Fitness magazine the 44-year-old trained five times a week using a mix of resistance training and danced-based cardio to mimic the People's Princess's slim silhouette.

As intense as the fitness program might seem, Simone—who has also worked with Anne Hathaway and Sandra Bullock—says her program is actually "a fun way to get your sweat on and burn calories."

It is clear from her grueling routine Naomi didn't take the nabbing the role of Prince Harry and Prince William's mom lightly. She recently shot down critics who said she wasn't perfect for the part. "Of course, people are going to jump and say 'she's not tall enough; she doesn't have the nose, the look, the voice, all of those things,'" she told InStyle U.K.. "But once I got past that, I liked the idea of the challenge."