The Cold Mountain beauty was convinced the actor would be serious and intense during filming of the thriller, in which she plays a United Nations languages expert, who tells Penn's secret service agent about a plot to assassinate an African dictator.
But she was thrilled when she met Penn and discovered he was a lively, amusing man with a taste for wild nightlife.
She says, "Sean is what you'd classify as an actor's actor. He's all about performance and , because he's a director as well, he's very, very precise.
"It was an unusual combination - Sean, Sydney and I - but that's what made me want to do it. Everyone thinks he's serious, but he has a sense of fun and play and was always calling me up after work and going, 'Come on, let's go out to this jazz club and get drunk.'
"So the film's given me a friend I wasn't expecting."08/04/2005 17:13