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 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 subtly break the fourth wall by addressing the actors by their names, rather than ad-libbing or improvising names for the characters.
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.
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.
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.
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 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.
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 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).
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". Denise Crosby (Tasha Yar) was in "Death Wish".
On April 18, 2016, ABC announced that they would not be renewing the contracts of Stana Katic and Tamala Jones for a ninth season. Because Katic is a such a fan favorite and, as the leading lady, a large part of the show's success, the reaction to the announcement was so negative that, in an almost unprecedented occurrence, fans took to social media and actively campaigned for the series' cancellation.
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.
Though they shared an engaging chemistry on screen, off-camera issues between Nathan Fillion and Stana Katic allegedly got so bad in the later seasons that they were required to attend couples counseling together.
The last of all of Nathan Fillion's starring (Regular) female cast-mates from Firefly will have made their appearance as of (5-2-16). That will be Jewell Staite. The others are Gina Torres, Morena Baccarin, and Summer Glau. The only male cast-mate has been Adam Baldwin who has had a reoccurring role on the series as an NYPD Detective.
The actress Toks Olagundoye, who plays Hayley, previously starred in a series called "The Neighbors," also on ABC. Her character, Jesse Joyner-Kersee, worked at a fictitious restaurant called "Salsa CASTLE." Most of the time, only the word CASTLE was visible on her name tag.
Four actors from this show have connections to the DC comics universe. Stana Katic (Beckett) is the voice of Talia al Ghul in Batman: Arkham City as well as Lois Lane in Superman: Unbound; Nathan Fillion (Castle) and Susan Sullivan (Martha) have voiced recurring characters on the Justice League animated series and select DC Animated Universe movies, most notably Hal Jordan/Green Lantern and Queen Hippolyta of Themyscira, respectively; and Molly Quinn (Alexis) voiced Kara-El/Supergirl in Superman: Unbound. Additionally, Adam Baldwin, who had a cameo as a detective on the show, has voiced a number of Justice League characters, as have Michael Dorn and Robert Picardo, who have recurring roles on Castle.
The season five episode "Castle: The Fast and the Furriest (2013)" aired the day of the Boston Marathon bombing. In light of the event, ABC chose to postpone the airing of the following episode, "Castle: Still (2013)", which deals with Beckett stepping on the sensor trigger of a bomb, and aired "Castle: The Squab and the Quail (2013)" in its place. Even with the switch in air dates, on the season five DVD set, "Still" and "Squab" are listed in the order they were originally intended to air, instead of in broadcast order like the other episodes.
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.
Recurring themes of the first three seasons and most of the fourth are the development of Beckett's romantic feelings for Castle, her determination to not admit and act on said feelings, and her jealousy of his interest in and pursuit of other women. Though no one explicitly says it to her, it's heavily implied, and can be easily inferred, that if her feelings about him weren't so strong, she wouldn't be as jealous as she's depicted.