Writing begins as an idea, a passing thought that catches the attention of the author and makes them stop and take notice. In the beginning, the author is just niave enough to believe that the idea was theirs. They find themselves clever and are proud of their unique way of seeing the world.
In time, the author grows suspicious. They wonder if the voices inside their head are really a part of them at all. They seem to know things that the author doesn't remember learning. They've observed things that the author has never witnessed.
The author shrugs this off at first, chalking it up to inspiration. He personifies it, calling it a muse.
If the author is a good little writer, he will finish the work without incident and continue to believe that it came from his own mind.
It's only when the author strays from his task that the truth is revealed. The voices will wake him, screaming their monologues, demanding to be heard.
You can only fight it for so long.
If a story needs to be told, it will find a way to bleed from your fingertips on to the page.
The story has a will of its own.