How often have you gotten mad at a book because a character did something he just wouldn’t do? Let’s call him Joe. After reading about Joe for a couple hundred pages, I feel like I know Joe pretty well. He’s a tough guy who loves the old farmhouse he renovated. If Joe suddenly decides to sell his house to developers who want to bulldoze it, it pushes me out of the story, and it makes me mad. It might even make me stop reading.
The last thing an author wants to do is kick the reader out of the story. To avoid that, the plot needs to feel character-driven: meaning, the actions of the characters have to be believable based on what the reader has read so far. If you hit a point where the plot and the characters aren’t gelling, one of them will have to change to match the others (or both may have to change).
Changing the characters involves going back and adding details to scenes to give a different impression of their behaviors. For small adjustments, this works fine. I do this most often to add a skill or a detail about the character’s past. Having it come out of nowhere makes it seem false. By adding hints of it earlier, I can make it more believable.
If you find that your characters have gone on a big tangent from your intended plot, changing the characters to match is much harder. It may, in fact, require re-writing whole chapters or most of the book. That’s a pretty drastic decision.
When faced with that option, you have to decide which you like better: your intended plot or the characters you’ve brought to life. I usually side with the characters. When I find that the choice I originally thought they would make feels false, I try to think from their points of view. What would they do in those circumstances? How have they acted so far in the story?
You don’t have to pick the first option that comes to your head. Usually, there’s another choice that works with your intended plot but approaches from a different angle. If you want to keep both the character and the plot the same, you’re going have to add a dramatic event that changes the character’s behavior. Joe has to have a reason to suddenly sell the house he loves, and the more out-of-character the behavior seems, the stronger the reason will need to be. I try to save this method for special occasions: if the characters change too often because of big, dramatic events, that’s hard to believe, too.