I go over stories on my head like a show, usually before sleep. One story can go on for months, advancing little each day. It's not linear though, it goes back and forth, I re-edit episodes, change what happened, insert and remove characters, review the background world, fuse stories, grow new stories out of the same universe as previous ones. Eventually, I write down a sketch that covers part of them, in a very poetic form of writing. I like to capture the essence first.
For comics, I care about writing what I will read later, set the mood, the dialogues, I don't care much about everything being completely detailed. I revisit it several times, then I think about how to translate into images, and this thinking can go many ways, until I decide on a page layout that covers some text, and then start to sketch compositions, poses, cameras, always trying to match the mood/essence rather than a dry approach. My writing goes hand-to-hand with my drawing, I don't try to separate them.
Except when I just write, then I write like a tempest, I can't stop to check anything, it's a river flowing. I usually stop when my hand starts to ache.