Listen to the Voices In Your Head...
I can't tell you how many twists and turns my book has taken because characters will suddenly be running off in a totally different direction than what I had planned. They take side trips that sometimes work really well. It's even happened with some of the secondary and background characters. I'll be writing a little walk on scene with them and BANG, I know their background and history and how it will play a key role in the main story.
Now this works out great for me since I'm planning a series of novels. Some of these secondary characters and their back stories, are going to become the main characters in some of my later books. It already happened with my first novel, which is still under rewrite. One of the secondary figures had lost a parent in a strange traffic accident that still has a number of unanswered questions hanging over it. I was going to make this part of my current work, but the book was becoming too long. So I've set that part of his story to the side for now. But, I've hinted heavily at that mysterious accident that made him become a cop, thus sowing the seeds of interest for that later book.
So be flexible when this happens to you. If the characters aren't behaving themselves, they may be laying the groundwork for later stories. And sometimes, you may even have to remove them from your current project altogether, because you have a totally different vision for them elsewhere.
My last piece of advice for today is this. If you start hearing other voices in your head, that are not anyone nearby or in your writing do the following. Make an appointment with a head specialist. Just remember to pause at GO and collect $200.00. You'll need it to pay for the session.
Enjoy and keep writing.