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.
In the recurring intro of season 2, Castle compares his and Beckett's relationship to "Turner & Hooch". Scott Paulin, who plays Jim Beckett (Kate Beckett's father) plays a role in Turner & Hooch (1989).
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 names, rather than ad-libbing or improvising names for the characters.
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).
Seamus Dever and Jon Huertas both had roles on the TV show NCIS (2003) as guests before playing their roles as Det. Kevin Ryan and Det. Javier Esposito on Castle. Darby Stanchfield (Meredith) also played Gibbs' first wife, Shannon, on NCIS in flashbacks.
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".
In the episode "The Final Frontier," the evil space creatures are called "Creavers." In Firefly (2002), 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.
Both Castle and CSI: NY (2004) have very similar guest characters, including CSI:NY episodes where Seamus Dever (Det. Kevin Ryan) and Arye Gross (M.E. Sidney Perlmutter) guest star on two separate occasions.
As a tie-in, the "Nikki Heat" and "Derrick Storm" books Castle writes are published in real life. The novels are written as stand-alone stories that don't require watching the TV series, but also contain in-jokes and references to events on the show. While the books are credited to Richard Castle, the ghost-writer's identity can be found in the author's bio in the form of an eponymous award that Castle has won.
Beckett was about 19 when her mother was murdered. In Castle: Rise (2011), it is mentioned Beckett is 31; since episodes appear to be set in the year in which they air, it can be assumed she was born in 1980. And in Castle: Always (2012), Beckett visits her mother's grave, the headstone of which reads that she died in 1999.
In season 07 episode 21 Castle and Alexis solve a murder aboard a plane. The Airline the two are flying with is called "Oceanic Air" just like the "Oceanic Airlines", to wich the plane belongs, wiches crashing has the events in the famous tv series "LOST" as a concequence.
There is a role in season 05 episode 10 "Significant Others", wich is called Billy/Billie Piper, wich is also the name of the actress, playing "Rose" in the first season of BBC's "Dr. Who", a beloved series among "Science Fiction Freaks" like for example Richard Castle.
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.