I've heard more than a few authors say that once their characters are fully developed they often will take over and drive the story and plot. Others prefer the characters, no matter how well developed, stay within the original borders of the story.

The preference may be dictated by the degree the author outlines their stories or if he just starts from the beginning and lets things happen from there. And certainly well developed characters in a series would govern any new story lines an author creates to avoid having them go against type.

Do your characters come alive and rule or does the story? Will a character cause you to change a story or plot in the middle of a book because it's something the character (as you've developed it to that point) would not do?