Tom Hanks—Cast Away (Hanks went from a slightly overweight businessman in the first half of the film to an emaciated castaway on a deserted Fiji island in the second—ultimately losing 55 pounds to play plane crash survivor Chuck Noland. Not only did he shed the weight in four months with a strenuous diet and exercise regimen, he wasn’t allowed to shave or cut his hair for weeks.)

Jared Leto—Chapter 27 (The normally buff and hunky Leto packed on the pounds to play John Lennon assassin Mark David Chapman. Rumor has it he forced himself to eat mostly unhealthy foods, including lots of ice cream, in order to gain close to 67 pounds. By the end of filming he has said it became difficult for him to even walk to the set due to the pain caused by the sudden weight gain.)

Gerard Butler—300 (Butler has never been hard on the eyes, but after preparing for his role as King Leonidas in the movie 300, he reached new heights of dreaminess. The star conditioned for four months, working out with trainers Franco LiCastro and Mark Twight, who created the “300-rep Spartan workout” to get a defined six-pack and rock-hard body. The sessions would reportedly last up to six hours.)

Renée Zellweger—Bridget Jones’s Diary (Zellweger's role as a hopeless romantic in this blockbuster hit required her to go from a size 6 to 14. In fact, she did it three times total for the franchise—gaining and losing around 30 pounds for each film! For the first movie, her diet consisted of doughnuts and pizza, but that left her feeling ill, so she worked with a nutritionist on the two sequels to safely gain weight by following a controlled high-calorie diet.)

Christian Bale—The Machinist (The handsome Brit normally weighs around 185 pounds, but dropped a shocking 63 pounds to play insomniac Trevor Reznik. Bale has said he eliminated almost all foods (according to People, he subsisted on coffee, cigarettes and an apple a day) and ended up shedding over 20 pounds more than was required.)

50 Cent—Things Fall Apart (It’s hard to imagine the usually buff 50 Cent losing copious amounts of weight, but he did just that by adhering to a liquid diet. Taking on the role of a football player battling cancer for the film, he dropped 54 pounds in just nine short weeks. In addition to his regimented diet, the rapper/actor ran on a treadmill for three hours a day.)

Mariah Carey—Precious: Based on the Novel Push by Sapphire (The gut-wrenching film Precious had the world buzzing, but not just for the gripping story. Carey’s transformation from glam to drab for the role of social worker Mrs. Weiss was hard to miss. Not only did she go onscreen with minimal makeup, but small details, like redness under her eyes and nose and upper lip hair, were added to enhance the character’s homely appearance.)

Cameron Diaz—Being John Malkovich (Barely recognizable, Diaz played Lotte Schwartz, a pet fanatic who tries out her husband’s portal to another world. There were no details in the script concerning her appearance, so she worked with director Spike Jonze to create Lotte’s look: transforming her normally blonde, straight locks into a brown, frizzy mess.)

John Travolta—Hairspray (Playing the cult classic character Edna Turnblad in the movie version of the Broadway musical Hairspray, Travolta was transformed into a heavyset…woman! To achieve the look, the film’s makeup department spent four hours each day covering him in padding, prosthetics, makeup and a wig.)

Charlize Theron—Monster (The typically toned Charlize Theron beefed up for her part as prostitute and serial killer Aileen Wuornos in Monster. Theron is said to have eaten doughnuts to pack on 30 pounds. She also had her eyebrows completely plucked out and her hair “fried” and thinned. To alter her complexion, layers of latex were added to her face and she wore dentures over her real teeth.)