Matthew McConaughey: An actor hasn't startled us this much since Christian Bale whittled away to skin and bones in The Machinist. The Magic Mike hunk dropped 30 pounds to star in The Dallas Buyer's Club, the real-life story about a man dying from AIDS in the 1980s who treated himself with alternative remedies he smuggled into the United States from Mexico.
Matthew Fox: The actor lost 40 pounds to become shockingly ripped to play a killer opposite Tyler Perry in Alex Cross. He trained for five months with the same man who helps Daniel Craig get into James Bond shape.
Aaron Carter: The singer posted a picture of his newly beefed up bod on Twitter, writing, "I've been at Johnny Wright's compound working on my mind, music, and body for a new album release!"
Taylor Lautner: The teen heartthrob went from cute kid to beefy babe as werewolf Jacob Black in the Twilight sequel. "The day I finished Twilight, I came home and started bulking up," Lautner told Interview magazine. "For New Moon, I'm 30 pounds heavier than I was in Twilight."
Rooney Mara: The up-and-comer has been spotted around town looking leaner for her starring role in The Girl With the Dragon Tattoo. According to a source, the actress is also cutting her hair supershort and doing Pilates workouts to achieve her character's "anorexic" look.
Christian Bale: The actor is no stranger to morphing for movie roles. In 2004, he lost more than 60 pounds for his skeletal role in The Machinist, and he did it again for the upcoming The Fighter opposite a very bulked-up Mark Wahlberg.
Jake Gyllenhaal: The actor looked more like a god than a prince for his role in 2010's Prince of Persia. Gyllenhaal said he studied Parkour, the French art of street running, to bulk up for the film.
Tom Hanks: "All it is is time and discipline," the Oscar winner told Time magazine of his role in 2000's Cast Away, for which he lost over 50 pounds to play a man stranded on an uninhabited island.
50 Cent: The usually ripped rapper shocked everyone with this pic of him looking dangerously skinny. He reportedly went from 214 to 160 pounds in just nine weeks to play a football player diagnosed with cancer in Things Fall Apart.
Robert De Niro: Shaping up for movie roles is nothing new. The legendary actor earned an Oscar nom for his role as terrifying tattooed rapist Max Cady in the 1991 thriller Cape Fear.