Jared Leto (Gained 60, Lost 40)
Credit: Everett

Rather than opting for a fat suit, Jared Leto packed on a reported 60 pounds to play Mark David Chapman, John Lennon's killer, in Chapter 27. The extra weight, he's said, "changed the way that I walked, it changed the way that I talked, it changed the way I felt about myself and the way people looked at me and treated me." He used that same rationale when he decided to drop to a dangerously low 114 pounds for his Oscar-winning role as Rayon, a transsexual with AIDS, in Dallas Buyer's Club. "I stopped eating," he's said.

The Bottom Line:
While the idea of letting yourself go may sound like fun to some people, Leto learned about the real medical consequences first-hand: While playing Chapman, he suffered from high cholesterol and gout. Even trying to put on weight after playing Rayon wasn't easy: "Your stomach gets so small that you can only eat a few tablespoons of something and you're full already," he's said. "So you can't go out and gorge, it doesn't work like that."