What's an example of an actor singlehandedly dragging a movie into being enjoyable?

The best example of this that I can think of is, of all things, Jim Carrey in The Incredible Burt Wonderstone. He takes a pretty bland comedy about a magician and makes it watchable purely on the strength of his performance. His scenes are by far the best and most interesting part of the movie, he's magnetic every time he's on screen, and it's clear that he had a ton of say in the creation and development of his character.

I'm sure that can't be the only example of this happening, just the one that comes to mind for me. What are some other cases of the phenomenon that you've seen?