Some actors just don’t appreciate their jobs. When a movie star quits his role in the middle of a shoot, or between sequels, it really puts the Hollywood machinery to work, as everyone involved scrambles to find a good way to replace them. Nevertheless, it happens all the time, particularly when actors feel like they’re better than the work they have been given. This has happened on big franchises like Batman and Star Wars, but also on TV shows like South Park and Community.
Sometimes, it makes sense, and the actor is just doing what is best for the film or television series. When Harrison Ford bowed out of Star Wars, or Keanu Reeves left the sequel to Speed, it was hard to claim that they were making the wrong decision. Likewise, you can’t really blame Michael Keaton for leaving the Batman franchise after his friend and director, Tim Burton, left the project. Other times, however, the actor is having an enormous ego trip, and the whole production has to suffer because of it. When Isaac Hayes left South Park, or when Vin Diesel left The Fast and the Furious franchise, a lot of people had to find creative and financially viable ways to work around those setbacks.