I try to get down some description of how the characters move about while writing the script - though most of it boils down to things like "Colin frowns" and "Lexie hits Colin" - so that I've got some idea of what they're doing while they're talking. I don't separate things up into pages until I sit down and do the thumbnails/sketches for the pages.
Sometimes, one page can cover two lines of script, and sometimes, it covers ten lines - it's all a matter of how much text I can fit on a page without it seeming cluttered.