Help! My Characters are Revolting!

BoysMom suggests a topic:

Sometime maybe the folks around here would like to talk about middles? Specifically, the plot going one way and the characters going the other.

So what do you do when your characters rise up in revolt? How do you handle it when your lead character looks up at you from the page and says, "Uhn-uh. Not gonna do it. Doing that would require me to be either stupid or desperate, and I don't think I'm that stupid, and I know I ain't that desperate."

The simple and common problem of characters revolting against the plot as they become self-aware is the reason why my filing cabinets are full of great beginnings that fade out in the middle stretch. For me, this usually indicates some fundamental problem with the plot that I hadn't thought through before I started the story, but which my characters were able to spot once the story was in progress, and now I have to go back to square one and start over.

Less often it means the plot is sound, but I've made the character too happy where he is to allow the plot to proceed. The challenge then is to find some way to force the character out of his comfort zone so that the plot can resume forward progress.

Fortunately, in complaining, the character usually gives me a clue. E.g., when a character says, "I ain't that stupid," well, there are a lot of ways to induce temporary overwhelming stupidity. Likewise when the character says, "I ain't that desperate," it usually means it's time for the hired killers to show up.

What works for you, and why?