Speaking at the Cannes Film Festival press conference for the political thriller, Watts said that she received the Fair Game script less than two weeks after giving birth and was asked by director Doug Liman to go to a training camp to shake off her "maternal state".
"Doug said, 'Get your arse into boot camp and toughen up lady'," Watts recalled. "Valerie is very calm and a quite delicate person, but she's got another side you don't know about that's rooted in this training."
She continued: "The first day, as they kicked me in the shins and threw me down I said, 'Ow' as you do. They were like, 'None of that'."
"They specifically said don't say 'ow' unless you need to go to the hospital," Liman added, noting that Watts came back "a changed character" after the training.
Fair Game, currently screening in competition at Cannes, recounts the true story of Plame and her husband Joe Wilson's clash with the US government after Plame was outed as a covert CIA spy by White House official Scooter Libby.
> Click here to read our Cannes review of 'Fair Game'