The screenwriter for Stranger than Fiction planned to have the professor help Harold Crick live. " Instead," says Helm, "I found myself writing Hilbert saying, 'No, you have to die.' I just found this character saying that, and I began thinking, 'Well wait a second, I don't want him to die, that's not where I want this to go.' But when he said, 'It's her masterpiece,' I realized that that's what that character would do."
Tom Perotta also talks about Little Children, and how the subplot with the pedophile was introduced when one character needs a ride – and it turns out to be with a person who is determined to get the pedophile out of the neighborhood. Perotta had never planned to write that.
I've had characters change on me. Most notably Don Cannon in Learning to Fly. He was supposed to bad to the bone – but he became pretty moral (for a guy who killed people) as the book went along. He simply refused to be all bad.