If I was making a D&D movie I'd have everyone with vaguely English accents. All except one guy who speaks in modern American English complete with modern slang and metaphors. The others would occasionally role their eyes or tell him to get serious and then he'd have a vaguely English accent for a sentence or two before lapsing into modern speech again.
I would also have one actor play/voice every NPC the way Deerstalker Pictures does. This guy is the DM of course.
The thing is based on a game after all, not on a book. So treat it like a game.
