The lengths to which Leto went for his latest role might seem just a tad extreme. To play John Lennon's assassin, Mark David Chapman, in J.P. Schaefer's film, "Chapter 27," Leto packed 67 pounds onto his lanky frame to better approximate the killer's doughy physique. By the end of filming, the extra weight had taken such a toll that he could no longer walk to the set. It's easily his most dramatic transformation to date.

"I'm not sure it was the wisest choice," he admits. "A friend of mine was recently going to gain weight for a film, and I did my best to talk him out of it. Just because you can lose the weight doesn't mean the impact it had on you isn't there anymore."

The abruptness of Leto's weight gain gave him gout and a doctor's recommendation to take Lipitor. But Leto says slipping into Chapman's skin was the best way to understand him. "The script didn't say, 'Page 1: You gain 67 pounds, and you're miserable for two months.' But as I started to research, I realized that the physical representation of this guy had so much to do with who he was."