Jack Black started going to the gym after getting horrified with the size of his ass in "Margot at the wedding"

"I was shocked to see the film when I saw my rear. I thought it would be a bit more shapely. They lied to me to get me to do the scene. They said, 'Oh, your ass, it's mouth watering. Let's get this on film before it's too late!' And I believed them. But it seemed like I had three asses somehow... My ass had double chins. That's not right. I've joined a gymnasium since the film. The next time you'll see my ass it'll be much more flattering". source