It's easy to think of filmmaking as a huge, impersonal machine run by studios simply churning out blockbuster films one after the other and collecting massive amounts of money for their pains. But you run the risk of forgetting that the entire movie system is made up of people, who are notoriously unpredictable. No matter how much they make arrangements for every contingency during pre-production, there is always the possibility, however slight, that something will happen to ruin all those carefully laid plans. And really, it's kind of scary when you think about how much money is depending on a bunch of fallible little humans. Sure, most of the time, things work out fine. Movies make it through the filmmaking process with their original cast intact. But sometimes stuff happens. Actors get hurt, or actresses get pregnant, or God forbid, the cast is disrupted by the serious but vague "creative differences" that always signify doom and gloom. And when these kind of things happen, production has no choice but to replace the actor. When they're lucky, the new actor has at least a little bit of time to prepare for such an important role, but sometimes, they don't have that luxury. In the case of the actors on this list, they have sometimes only a day or two to get themselves into the swing of things.