According to La La Land director, Damien Chazelle, Watson left the project to play Belle in Beauty and the Beast. Watson later stated that the role in Beauty and the Beast seemed like it would have reached a larger audience and had a bigger impact on society. Boy, was she wrong.
Matt Damon, Avatar
Damon likes to make the joke that he "bailed on" the production of Avatar. In reality, he only denied the role of Jake Skully because he had a busy conflicting schedule. Damon didn't even want to decline the role, in fact, he was excited about it.