So...how to explain this bizarre phenomenon to someone who's never experienced it?
Think of your best friend or your spouse (or maybe they're one and the same). Think of how well you know that person. And then imagine that someone else started to tell you something about them. Maybe someone you work with tells you they saw your husband at the mall buying a Fox shirt. But you happen to know that your husband despises Fox to such a degree that not only would he never, ever be caught dead wearing one, he would never even be seen in that store. Right away your brain says, "Nope, uh uh, you must be mistaken." It's the same thing with your characters.
If you're really a character writer, you spend hours with them, delving deep into their pasts until you know them. You know what they like, what they hate, what they're afraid of, what makes them tick, their strengths, their weaknesses, everything, anything. You know more about them than anyone would possibly need to know--way more than what will ever end up in the book. That's what makes them real to you. And that's what gives them their voice.
So when you write a scene with your characters, it's almost like they're in control. You may want two characters to get along and be best friends. But if you know that one character hates anyone who drinks soda (maybe their dad was a dentist and tormented them with pictures of cavity rotting teeth until they were traumatized by anything sugary) and the character they're supposed to be friends with has a six-pack a day habit, guess what? They're never going to get along. You may be able to get them to play nice occasionally...but best friends? Out of the question. And if you try...they'll argue with you. The scenes will feel forced. The dialogue will sound contrived and unnatural. And all of it will be totally unusable. Why?
Because when you know someone really well you know what they're going to say. When you call your best friend to tell her you're getting married you know how she's going to react. If she's been secretly planning your wedding with you for the last three months she's going to squeal and cry and talk wedding plans. But if she's going through a bitter divorce and has decided that marriage is for the birds she's going to lecture you and try to talk you out of it. And if she said or did anything different you'd know she was lying to you. It's the same thing with dialogue. When you write a character you really know, their dialogue comes as naturally and effortlessly as if they were whispering it in your ear. Whole conversations will simply write themselves and you'll look at your pages at the end and think, "Wow, where did that come from?" It came from the characters. They were talking for themselves.
So do I actually hear a little voice in my head talking to me? No. That would be disturbing. But my characters do speak to me. They boss me around and push me and fight me for control every step of the way. Does that mean I've lost control of my own story? Of course not. I'm still the writer. I can still manipulate the plot all I want. But I can't always control the way my characters will respond to what I throw at them. So sometimes I have to change what I put them through. Or I have to change their past. But in the end, it only makes my writing stronger, because it makes it real. It's not some perfect fairy world where everyone is a robot and responds to my every command. It's real life where people are different and unpredictable and exercising their own free will. It's crazy, surreal, and incredibly aggrivating. But I wouldn't have it any other way.