Re: Castle: Season 3 (Nathan Fillion, Stana Katic) - Discussion & Spoi It took me the whole episode before I realized that the mousy assistant was played by same actress who plays the werewolf's sister on BEING HUMAN. I knew I'd seen her somewhere before . . . Fun to see Castle and Beckett discussing "shippers" and such. But, of course, the best bit came at the very beginning: "Who would want to kill a writer?" "Oh, so many reasons . . . ."