Nathan Fillion gives TV Fanatic a bit of Castle scoop in this interview, speaking on a fun future guest star and the topic of his characte's father.
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Castle Quotes

Rick Castle: That thing in the shower. That is delightful.
Kate Beckett: That is not a routine. That's two naked people singing when there's no one around to see or hear it.

Button up kitten because we're going home.

Beckett