I talk to myself. I talk myself through the plotline of a story, I speak the dialogue out loud - often repeating myself until it sounds right - I work on plot-issues by literally talking myself through them.
I act out the story, pretty much, playing the parts of all of my characters - the dialogue, as mentioned, but also the choreography of the fight-scenes, the facial expressions of the characters, etc., etc. I've always done this - tell stories out loud to myself, I mean. Some people sing in the shower - I tell stories.
I do my best not to do this in public where other people can see me - mostly because I choreographed a fight-scene at a bus stop once, and turned around to find a very concerned-looking old lady watching me - but as soon as I have some peace and quiet, I can't seem to NOT do it.