Jake Gyllenhaal (gained once, lost once)
Credit: Everett

The actor shocked fans when he appeared on a red carpet last fall with sunken cheekbones. It turns out he'd been filming the upcoming crime drama Nightcrawler, in which he plays a guy he calls "literally and figuratively hungry." As for how he did it, he said, "There really was no technique" – it was just for the love of the film. His look was quite a change from the beefy, action star body he got for 2010's Prince of Persia. (His training for this included parkour, running in a 20-pound flak jacket and six small, protein-packed meals a day). "I'm a really athletic person, I'm very physical, and to be able to, you know, get paid and to do it as your job was just fun," he's said.

The Bottom Line:
When it comes to getting back to regular size after bulking up the way the actor did for Prince of Persia, it's a myth that all of that muscle would have just turned to fat if he quit the workouts cold turkey. "Losing muscle and gaining fat are two separate processes," says Bacon. The trick to getting back to regular size is balancing caloric intake and output.