Some of what I'm about to say might sound a little crazy to someone who isn't a writer. However, to most of us writers, it's probably going to be one of these moments:
As writers, we need to be able to create characters. That's essential to our trade. Some characters come to your imagination ready made. Others come with a little sticker that says, "Some assembly required."
You build their physical appearance, start to build their personality, their backstory, and other essential things. Then you set them loose on the page. You usually have a feeling like a proud parent just about then. And you generally have a plan for them in mind. But then they have the nerve to go and completely disregard all of your well-made plans!
As the writer, you might think you're in charge of the story and it's cast. It's possible you might find it a bit of a surprise when you discover that isn't exactly the case.
They've become so lifelike that they've practically started running the show, as far as your novel is concerned. If you try to force these kinds of characters into decisions they don't want to make, then you may regret it in the long run. There are a lot of times when the changes made because of these characters end up making your novel so much better. Plus, these characters usually know what they want and sometimes it's not so easy to change their course once they start on it.
Now, this isn't to say that you shouldn't keep a good eye on the big picture of your plot. If these characters start making decisions detrimental to your plot line (now this is different from changing plot points), then it might be time to reign them in. I haven't had this happen too many times, but it is an occasional problem. Sometimes things are better the way you'd initially planned them. Don't be afraid to make your characters behave. You are the writer after all.
But more often than not, these characters make good decisions that enhance your story. So let them run wild once in a while! Who knows, it might do your novel a lot of good! Don't be afraid to go with the flow and let your story and characters take on new directions and dimensions. I don't think you'll regret it.