Naomi Watts wasn't allowed to complain about pain on her new movie.

The 42-year-old actress - who has sons Sacha, three, and Kai, two, with long-term partner Liev Schreiber - was sent to boot camp to prepare for her role as real-life former undercover CIA agent Valerie Plame in new movie 'Fair Game' and admits she found it tough.

She said: "The director, Doug Liman sent me off to boot camp. He thought it was important I had the same tough exterior in the movie as Valerie, and that I looked too soft and maternal.

"I'd just had a baby, Kai, on December 13 and I read The Script on December 28. We were filming in February.

"As Doug was walking out of the boot camp, they kicked me in The Shins and threw me to the ground. When I went, 'Ow', they said, 'Don't say that again unless you need to go to the hospital."

Because she went back to work so soon after giving birth, Naomi admits she doesn't think she was really "up to" such an intensive physical project.

She added: "I did paramilitary training and I was allowed to have Kai every few hours to feed him. But otherwise, I couldn't have anyone else there.

"It was intense and not something I was really up to. I did incredible things that I'll never get to do, or wish to do again, you know, setting off explosives, ramming cars without a seatbelt or a Helmet."