From "Undead Again"
Castle: THAT, is a Zombie. (strides away and annouces to the precient) The Killer's a Zombie.
Ryan asks Castle if he really believes in zombies
Castle: No. You what I DO believe in? Driving Beckett crazy.
Castle makes Zombie noises behind Esposito
Esposito: Stop it. Don't do that, you're creeping me out.
Beckett: And that wall I was telling you about, I think it's coming down."
Castle: Well, I'd like to be there when it does.
Beckett: Yeah, I like you to be there too.
Castle: Only without the Zombie make-up
Beckett: I don't know, I kinda think the Zombie make-up suits you Castle.
Castle: Yeah, I make it work.