The artist just draws the book. He doesn't have to write it as well. So long as there is a detailed break down of all the panels there won't be any screw ups. And even if there are a few minor ones, if we can get some one like Travis Charest to do it nobody'll care.
McFarlane takes half an hour a page? Unlikely. More like a day a page. But it doesn't matter anymore since he doesn't pencil or even ink any more.
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Mess with the best, die like the rest.