Star Wars: Episode III: Recently producer Rock McCallum & visual effects supervisor, John Knoll, talked with fans at the Official Star Wars Website about the progress of the film's production:

"Christopher Lee has arrived in Sydney, but he hasn't started shooting yet" whilst there is a character coming from the expanded SW universe (ie. in the novels), but "those scenes won't be shot until early next year, so there hasn't even been any casting yet". Meanwhile Knoll says the Stormtroopers will "still be CGI and they'll be noticably different", there "are currently no plans for a CG Chewbacca" and "Wat Tambor is back and badder than ever."
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Thanks to DH for the scoop!