Let's think about this - that's just kind of crazy, isn't it? These people don't really exist. Admit it. They don't. No one is taking over your story but you. Only you're writing it - unless you're working on something with multiple authors.
So let's get one thing straight - you can't sit around blaming your characters for things that go wrong. Stop it right now. YOU are in control, and if your story starts going off on some tangent and you think you have to follow because your characters are leading you there, well, you're just being lazy.
Sherrie Peterson wrote an excellent post the other day titled Who's the Boss? Here's a bit of her experience:
Well, I did plenty of flailing. I wrote this weird, melodramatic, dark crap that I hated. I kept thinking if that's where the characters wanted to go, didn't I, as the author, have an obligation to follow them there?
I've decided that way of thinking is wrong, at least for me, and certainly, for this story. My characters are NOT the boss of me.
When it came right down to it, the problem was lack of confidence. I didn't think I was good enough to write the story the way I wanted it to be written, so I fell back on easier solutions. I gave the characters stupid obstacles to overcome and made it too easy for them, for ME, to find a way out. And it was boring. I hated the story so much I put it to the side and worked on other things.
There you go. Sherrie admits that her problem with the story was lack of confidence - and that can easily stray you into lazy-land when it comes to writing. No matter how you look at it, writing a good novel is hard. I remember writing this particular paragraph in my current WIP, a novella about Cinderella:
Rose appeared from the crowd next. Her cheeks mimicked her name. Her fluttering movements made her emerald green dress swish along the floor, and the sound seemed to travel up the material, all the way to Rose’s throat as she curtseyed and said in an airy, billowing voice, “Your Royal Majesties, I am honored that you have allowed us to speak today.” She stood in front of Lucy and Edith now, her tall frame a dark spindly tree between their bright dresses.
I remember thinking at the time - wow, I love Rose! She speaks clearly to me. Introducing her was easy. But why? It's not because she's a living being taking over my manuscript, that's for sure. I remember feeling tempted to put Rose in more scenes. She's Cinderella's stepmother, and in my story, she's only in one scene. She's a strong character, but that doesn't mean I'm going to drop her all over the place whenever I hit a rut. It's tempting, but no.
Remember that you're in charge of everything. Not critique partners. Not characters. Not all those rules you read about. If you run in a direction that feels like your character is "taking over," understand that, yes, creating a character that comes to life in your mind is fun, but it's still you that created the character, and you have the power to change direction. You certainly don't see Bella Swan's name as the author of Twilight.
Like I said in Sherrie's comment section on her post, I think we reach a higher level as a writer when we see our work not as set in stone or something out of our control, but as something fluid and flexible and something where we can break the walls down and rebuild and rewrite without ruining anything. When we reach that point, we're really writing, not just spitting out stories in our head.