But until it's actually written, you don't know what they're wearing, what time of day it is, what room/sort of scenery they're in, what emotion is really going on between them in that scene... (And often the author won't know this either, since they are conditional on other things which happen in the writing during the lead-up to that scene).

THIS!!!! And I love your idea of letting the artist have a month after the fic is finished to illustrate the last half of it. :D