Jack Black was left horrified by the sight of his own bare butt in new movie MARGOT AT THE WEDDING - because he didn't realise how terrible it looked. The chubby funnyman was reluctant to drop his pants for a scene in the film but producers won him over by telling him he had a lovely bum. Then, when he actually sat to watch the film for the first time, he realised he'd have to work out before getting naked onscreen again. He says, "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."