In Writing Magazine (a publication of Weekly Reader), I found an interview with Stephen King in which he discusses how plot and character interact. The question was whether he works out the plot of his books beforehand. Here's what he said:
"No, I never plot beforehand...I have a general story idea--a situation. That's where I like to start. Then I let it play out. And that always works when you're honest about what characters would do in a given situation. If you start to lie then things wander off-course. That is, if you start to make characters do things because it would be more convenient for you."
He gave this example:
"I'm working on a story right now where I really didn't want this lady to call the cops. Because it would be inconvenient for me. I would have to write about police procedure and bring people into the story that I didn't want there. But the situation she was in, any sane person would call the police. So I let her call the police. And it was fine."