(Photo credit: Karwai Tang/Getty Images)

At first, Naomi Watts wasn't sure if she wanted to play Princess Diana in the new film, "Diana," but once she accepted the part, she really committed to the role.

According to New York-based trainer Simone De La Rue, Watts, 44, did a lot of resistance training and dance-based cardio.

Watch Naomi Watts in the 'Diana' Trailer

"[It's] a fun way to get your sweat on and burn calories," she told Women's Fitness magazine in the UK.

De La Rue, who also reportedly works with Sandra Bullock, Matt Damon and Anne Hathaway, said she encourages clients to work out with her for three months.

"When working towards a specific goal with a client, I encourage them to train five or more times a week, for one hour a day," she said.

Photos: Princess Diana Through the Years

It worked. However, Watts did have some reservations about playing the royal icon.

"I do care deeply about how the princes feel, of course," she said. "There was actually quite an eerie moment in London when I did see Prince William in a restaurant. I got very nervous, because if I caught his eye, I didn't want to interpret a look and think he had a negative feeling about this idea, so I made sure not to look."