The late comedian Mitch Hedberg had a joke in his act about how every stand-up comic in Hollywood has to be an actor, at least in some capacity. Just being a comedian, even a great one, isn't enough. And it's one of the few professions that forces this kind of duality. As Hedberg put it, "It's as though I was a cook and I worked my ass off to become a good cook and they said, 'All right, you're a cook... can you farm?'" It aptly describes many comics' aversion to getting wrapped up in the movie business. After all, if you're a stellar comedian, why not simply continue doing what you do best and leave the acting to the professionals? If only there were no truth to Hedberg's joke, and comics weren't pushed into being actors, the general public would be spared a massive amount of big screen stinkers. After all, not every comedian is going to have the results of a Steve Martin or Robin Williams, two stand-up icons who made the transition to movies look remarkably easy. Nor should you expect all of them to have the crossover success of Russell Brand or John Cleese. But they should at least be mediocre... right? Whether measured in box office success or critical praise, a lot of them were failures. There's just no nicer way to say it. The following laugh-makers tried to make a splash in the Hollywood pool but instead slipped off the diving board and cracked their heads on the wet cement below.