There's a reason the sex scene between Sienna Miller and Hayden Christensen in Factory Girl looks so real, it is. The pair had a brief fling during the month long shoot, and when it was time for reshoots six months later, they apparently picked up where they left off. New York Daily News says:

When it came time to shoot the love scene, word is the former lovers dispensed with the flesh-colored socks and pads favored by actors on less familiar terms. And, once the cameras were rolling, the two 25-year-olds fell into old habits..."It's not simulated," an insider tells us. "They're really doing it."

This is probably true because Sienna is famous for being an attention whore and her vagina is like Ellis Island when she gets on a movie set, so the only real mystery here is who Sienna picked last for the on-set gangbang. Probably the Mexican paint crew. Like Share Bear says, "Do your share of sharing!"

NSFW scenes from Factory Girl: