Although I don't usually work up thumbs to this level of detail, it's a worthwhile exercise to get obvious visual problems sorted before going to finished page art. Between the first and second draft of the script the first speech balloon on page 2 here suddenly became an orphan. Not good, and it obviously needs to be moved up into the final panel of page 1.
This is also where I try to work on character design. If you are going to have to live with these guys for months, or in some cases years, you've really got to be comfortable with them. What do they look like? What clothes would they wear. Haircuts? Body language? No amount of character sketching can solve all the problems - this is the stage where each character has to actually fit into the story. Its worth trying out different stuff, and so here the visual aspect of each character is still very fluid, and they may change quite a lot from panel to panel until I arrive at a stage where I am completely comfortable with their visual appearance.