Horror movies need to be careful when it comes to casting, as many great films have an unfortunate fly in the ointment thanks to one cast member that doesn't quite gel with everyone else. Because characters in horror movies react to such extreme things, it's very important to make sure the entire cast is on the same page when it comes to the general tone of the film. Sadly, even the best horror movies can fall prey to a single actor not understanding the assignment and bringing the entire effort ever-so-slightly down.