Renee Zellweger totally transformed her body for both 'Bridget Jones' movies. She piled on roughly 28 pounds for each film.
For the first movie, Renee simply stuffed herself with as many donuts and cheese pizzas as she could. For the second, a nutritionist helped her gain weight with a more controlled high calorie diet.
Renee revealed that it sounded "like heaven" to be able to "indulge all your fantasies about over-eating. Fantasies about non-stop chocolate consumption or your fantasies about ordering the pizza and the spaghetti and the garlic bread." However, in reality, "After a week your glucose levels are going crazy. You're up and down and all over the place. It doesn't feel good, and no one wants to hear that, but it's the truth."
Regarding losing the weight Renee said, "It was horrible," and that "the first lap around the track I felt like I had a toddler on my back."
Click through for more startling examples of stars who gained or lost massive amounts of weight for movies.
Remember, these are extreme cases -- Don't try these dangerous diets at home!