Why do people always tell actors to break a leg?
Because every play has a cast.
Listen up, buttercup! The joke is play on words, see? When people tell an actor to "break a leg", it means "good luck." But the second part of the joke uses the word "cast", which means the actors IN the play but ALSO what you wear when you break a bone. That's why it's so funny. You probably have broken a leg or two. You seem clumsy.