Unless you're schizophrenic (or a writer, and is there all that much difference, really?) you may be a little creeped out by this: most fiction writers, at least from time to time, hear voices in their heads. When we write dialogue we are not making up conversations so much as transcribing the ones we hear in our minds.
I guess it's a little like the way we dream at night. Our subconscious minds slip their leashes and raid the data banks of our memories for odd bits and then string them together to make good dreams, bad dreams, or downright weird dreams. It's not something we do on purpose. Generally, we wake up shaking our heads and muttering, "Where on earth did that come from?"
That's what it's like for writers. We can get so caught up in our own stories that sometimes we feel like we're watching them, not creating them. We don't plan for the protagonist to say something wise or funny or shocking; we hear him say it and then we simply write it down. Yes, we've created the characters and the situations, but our story people often surprise us and drag us with them into wholly unexpected situations. Like the dreamer, we're not really in control. And yet our stories, like dreams, are coming straight out of our own minds.
I don't know if most writers are born hearing voices or if it's something we just pick up. But I can tell you that when an imagination is exercised daily, it becomes stronger. Writers are often asked if they're afraid of running out of ideas. We aren't, any more than a track star is worried that a long, hard run will use up all of his leg muscles. He might tire and need a rest (writer's block) but he'll come back stronger because excersize doesn't deplete muscles, it builds them.
This morning when I should have been doing laundry, thinking about which suitcases to pack, and getting otherwise organized for my trip to Reno, I've been working on a romance novel. No, I'm not under a deadline; my editor doesn't even know about this project (although I'll tell her in a couple of days). But when I woke up this morning the hero of my story was talking, and because I was afraid he wasn't going to repeat himself at a more convenient time, I hurried downstairs to my office and began taking notes.
He's quiet now, so I'd better get started on that laundry.