Both at the same time I reckon, you don't have to draw charatcers out fully but doing quick sketches with notes will help you visualise people and places better as you wright. A picture paint a thousand words as they say. You could do an an entire script all at once, I've tried this, but I've found it to be quite a labourious process. You could do a storyboard, quick sketches of your panels/pages to get an idea of how they'll be layed out and have the speech written next to each page and use that as a 'script' . Alterentaivly you could type/write it up in movie script fashion and draw doodle next to your typing. Either way I think both at the same time is a good idea.