I’ve often thought that a candid fly-on-the-wall documentary about the production of the Harry Potter films would be considerably more entertaining than any of the lackluster pictures themselves (Alfonso Cuaron’s excellent Prisoner of Azkaban duly excepted). Alan Rickman’s recent diaries suggest that the sets were unhappy, frantic places where actors were seldom allowed to create memorable characters and where the focus on the juvenile performers meant that one of the finest British ensemble casts ever assembled often functioned as little more than expensive set-dressing.