Ain't It Cool News has posted an account from someone who has apparently seen some footage from the film version of Harry Potter and the Prisoner of Azkaban:
Gary Oldman as Sirius is as described in book - really greasy long black hair and shabby clothes. The werewolf is enormous and the different heads for the transformation are quite horrific. The animagus Sirius is also huge and effective, with someone inside controlling the animatronics.
Read the rest (yes, there's more!) at the link.