So I asked kickinpants "Do you see pictures of the action in your story either before or during the uhh act (of writing, I mean; I know you don't do yaoi) or do you hear words?"
Who answered thusly and asked me in turn.
I usually/ used to see pictures and hear dialogue, but not when actually writing. In the bath (when I had a Japanese bath, she sighs) or when falling asleep or waking up, or when listening to music; all times when I'm not, for whatever reason, the full-scale Me. (There's a whole entry waiting about me and music. I never use it as background precisely because it sends me off elsewhere. I listen to it lying down, of necessity, and that of course is next-door to falling asleep or being in a bath.)
Getting that vision into words- sometimes, just holding on to that vision- is the problem. Best-case scenario, the not-Me horizontal head movies provide a kind of master plan that light-of-common-day vertical Me in front of the compy just follows in its light-of-common-day fashion, so that the amorphous vision translates into precise words. This is sometimes helped by an intermediate exercise- walking. Vertical and moving is a state close to horizontal and listening to music, only it's a lot more rational. Plot points get worked out then and problems of phrasing. Back in uni I used to pace the house during essays, up and down the long hallway and into people's bedrooms, occasionally stopping to talk to mirrors. (This is why I could only write during the day when my family was out.)
Then there's that off-and-running thing. I don't know what happens then either. It's a kind of personal silence, I think. All that exists are the words on the screen and they dictate what happens and where we go next- or rather, they usually just go there and the fingers try to keep up. It's very nice, because I get to edit in next day and then do it again.
And I can only say, there's nothing drearier than trying to write fics when the only person doing it is the vertical person at the computer. That person can write reviews and sometimes articles (though with no felicitous turns of phrase.) She can't write stories at all.