When Stana Katic first met Nathan Fillion, she had just gotten out of hair and makeup for the final callback between her and one other actress, and realized she was wearing a blouse that was too long. She asked for a pair of scissors, saw Nathan Fillion getting coffee, and greeted him. He asked if she wanted to run lines, and she replied, "No, but speaking of lines, can you cut a straight line?" To which he replied "I can try." And so he cut her blouse shorter for her. The producers saw this and she got the part.
In the Nikki Heat novels, Richard Castle writes about the shadowing of Detective Heat by a writer by the name of Jameson Rook. In chess, "castle" and "rook" are interchangeable names for the same piece. To begin with, Castle's name was a parody of the name of the real best-selling novelist 'Stephen King', whose surname is also a chess piece.
Most people notice the prominent in-jokes/references to Nathan Fillion's previous TV series Firefly (2002) however, there is another, lesser known recurring reference: Whenever the subject of comic books comes up, Beckett expresses an admiration for the works of comic book writer/artist Frank Miller. This is most likely a subtle recurring in-joke/reference to the fact that Stana Katic, who plays Beckett, previously appeared in Frank Miller's directorial debut The Spirit (2008).