Ryan Maddux wrote:The third my MC took over and wrote a story I couldn't comprehend, and argued with her quite a bit in rewriting. Darn near me quit writing, as it was a weird experience.
That's happened to me, but for me, it was a side character who took over the story. She was supposed to have a small role, and decided that instead she would be the protagonist and make the story entirely about her own issues. I wasn't experienced enough as a writer to keep her in her place, so she took over & destroyed the story I was meaning to tell. I think she'd be great in another story, but I would have preferred she wait for her own book. 
When I write, I vanish. I don't care if there's music or no music, though I find sometimes that music can alter what I'm writing, sometimes for the worse, as I'll find myself writing to the mood of whatever is playing. So it's often better for the story to turn it off. Anyway, I vanish, as in, I blink, and twelve hours has gone by. But like Ann, this process can be shattered even by a moment in the real world. It's as if I'm creating strings of thoughts in my head, and they're leading me somewhere -- and someone poking in at the door expecting me to answer even a minor question shatters the strings, and within seconds I lose everything. But if I'm left alone, they build and build, weaving something in a place well beyond conscious thought. I think following the strings is the creativity? And knowing your story well enough to turn off the music and tell the side character to wait for her own book is the common sense. 
I have no idea if that's the normal writing process.