 Each episodes "cast in order of..." credit is different depending on the  plot of the episode. "Witchsmeller Pursuivant," for instance, introduces  the cast "in order of Witchiness," and "The Black Seal" introduces them "in  order of disappearance." "The Black Seal" also lists Tim McInnernys  character as "Percy, a Poisoner" (watch the episode to find out why)