6. Natalie Portman and Mila Kunis
When you're playing a mentally imbalanced ballerina, you need to ditch the chocolate bars for a bit. Uber-competitive ballerinas played by Mila Kunis and Natalie Portman were practically twigs, so it was necessary that they both drop 20 lbs. on the double. And while 20 lbs. might not seem like a ton, you'll have to remember that both actresses are impossibly tiny to begin with.
5. Christian Bale
You probably know Christian for his beefed-up roles in "The Dark Knight" and "The Fighter," but when he played Trevor Reznik, a psychologically tortured man in "The Machinist," Bale reportedly went from his traditionally 185-pound frame to weighing in at around 125 pounds. The change on his 6-foot-tall body was staggering, making Bale look totally emaciated. It was later revealed that while the director of the movie asked Bale to lose some weight for the role, he didn't condone the 60-lb weight loss. Now that's what I call dedication!