Bradley Cooper, is that you?!

For his portrayal of real-life Navy SEAL Chris Kyle, the 40-year-old actor followed a "strict, crazy diet" to pack on about 40 pounds for his newest film, American Sniper.

"Bradley used his own trainer, who was busting on him. I think he was working out four hours a day for several months," the movie's producer, Jason Hall, revealed to 'People' magazine.

bradley cooper

Bradley on the red carpet (left) and in American Sniper (right). (Photo Credits: Getty Images)

"He was determined to do it naturally, he didn't want to use any hormones or steroids or anything," Hall added. "He was just very systematic about it and took his trainer with him wherever he went."

But Bradley — who recently received his third consecutive Oscar nomination for 'Sniper' — isn't the first star to gain major buzz for drastically changing his physique for a film.

Click through the gallery below to see nine more stars who've transformed their bodies for movie roles!