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).
In scenes at the precinct, when Nathan Fillion, Jon Huertas, and Seamus Dever interact with background extras whose characters are meant to remain nameless (for example, perhaps most notably, recurring extra L.T. Tolliver), they just address the actors by their real names.
In the episode "The Final Frontier," the evil space creatures are called "Creavers." In 'Firefly,' the evil characters are the "Reavers." Castle also teases Beckett that he doesn't understand her fascination with a show that was on for "One season." 'Firefly ' was on for one season and has a huge following at comic book conventions all over the country.
There have been multiple guest appearances on Castle by actors from different Star Trek series. Michael Dorn (Worf) was Beckett's psychiatrist, Tim Russ (Tuvok) plays Dr Malcolm Wickfield, Penny Johnson Jerald (Kasidy Yates) plays Chief Iron Gates, David Burke (Steven Price) appears as Chief John Brady, Jonathan Frakes (William T Riker) directed the episodes "Kill the messenger" and "The final frontier", Armin Shimerman (Quark) played the prop maker Benjamin Donnelly in "The final frontier", Robert Picardo (The Doctor) appeared as Doctor Death, and Ethan Phillips (Neelix) was in the episode " The final nail".
When Castle and Beckett's relationship is purely platonic, he addresses her by her surname; after it becomes romantic, he begins to address her more frequently by her first name. Conversely, Beckett addresses Castle almost exclusively by his last name, calling him "Rick" only a small handful of times and rarely, if ever, calling him "Richard".
At some point in season two (possibly while Beckett was with Detective Demming, given his jealousy of their relationship), Castle creates, or at least adds to, a bucket list with "Be with Kate" as his number one goal.