Actor Allusion: Like his actor, Jon Huertas, Esposito is a military veteran.
The writers are excited to make shout outs to Nathan Fillion's Firefly days and he has no problem obliging their requests, even adding in little ones himself when appropriate. See the Trivia page for examples.
Alexis: Didn't you wear that, like five years ago? Castle: So? Alexis: So, don't you think you should move on? Castle: I like it.
In one episode, a minor character jokingly refers to Castle as "Jason Bateman". Castle retorts that their similar looks got him out of a traffic ticket once. This actually happened to Nathan Fillion in real life. A similar joke is made in "An Embarrassment of Bitches", where Castle is mistaken for Jason Bateman by some paparazzi. Castle complains that "this is getting old," much to Beckett's amusement.
"Nanny McDead:" Castle reminisces about the nannies Martha had watch over him back when he had yet to discover his talents for the art of writing, for whom he bestowed a wonderful range of other identities, one of them being "completely irresponsibly middle aged women who instead of looking after me while you were acting like they were supposed to, instead watched daytime television." The upside of it all was that he "got the plot of (his) first novel by watching One Life to Live". Nathan Fillion's first TV role was Joey Buchanan on the soap opera One Life To Live. Subtly reprised by the episode, "One Life to Lose".
Erick Avari cameos in an episode centering around a mummy's curse.
"Headhunters" opens with Castle playing with action figures, the exact same way the pilot of Firefly (Wash) did. Notably, this is the episode guest-starring another Firefly alumni, Adam Baldwin, as - what else? - a gun-toting, ruthless gang cop. In exchange for allowing Castle to follow him on a case, Baldwin's character forces Castle to give him his brown coat.
In "Murder, He Wrote", one of the Portmanteau Couple Names Castle tries out for himself and Beckett is "Kate-ick" — which isn't that far off from 'Katic', as in Stana Katic.
"The Final Frontier":
The episode begins with Beckett approaching Castle to tell him there was a murder at a sci-fi convention. His reply? "Shiny."
The main character on Nebula 9 is Captain Max Reynard. No small resemblance to Captain Mal Reynolds.
The "Silk trigger active return bolt laser pistol" from the Firefly episode "Heart of Gold" can be seen on the table of Benjamin Donnelly, the Sci-Fi tech dealer Castle and Beckett encounter (who, by the way, was played by Armin Shimerman, AKA Quark).
Nebula 9's astern (as seen as a blueprint on the fake screens in the fake ship) very closely resembles that of Serenity, the ship Malcolm Reynolds flies in Firefly.
Played with somewhat in "Reality Star Struck" which guest-stars Gina Torres, who played Zoe in Firefly. Her character and Castle have little to no interaction whatsoever, and with the exception of Gina Torres playing yet another strong-willed woman, no overt allusions to the show come up throughout the episode.
In "The Squab and the Quail", Ioan Gruffudd plays a genius-level inventor, billionaire and all-round innovator. You might say he's a Mr. Fantastic, even.
In "The Human Factor", Jared Stack mentions the local Counter-Terrorist Unit. Actor Carlos Bernard memorably played CTU Agent Tony Almeida in 24.
In "Limelight", Alexandra Chando plays a child-star-gone-party-girl called Mandy Sutton. The acress' last TV role was a character whose name was Sutton Mercer.
Not quite an allusion, but in "Number One Fan", when Castle is shot and knocked unconscious for a brief time, he wakes up shouting "Cheeseburgers!" while sitting bolt upright. This parallels a moment in the Buffy comics where Buffy, under the influence of a "Sleeping Beauty" Spell, shouts "Cinnamon Buns!" while doing the same thing.
A subtle one in the Heat Wave book: in the series, Castle's judge friend is played by Dan Castellaneta, better known as the voice of Homer Simpson. In the book, Jameson Rook's judge friend is named... Horace Simpson. Nikki even expects him to say, "D'oh!" when she beats him at a poker game hosted by Rook.
One of the suspects in Heat Rises, the third book, mentions as his alibi that he was at home, because he had rented Quantum of Solace, and mentions that Detective Heat would make a good Bond girl. Stana Katic (who also plays Kate Beckett, the inspiration for Nikki Heat), played the Canadian intelligence agent at the end of Quantum of Solace Corrine.
In "Deep Cover", Martha is reminiscing about the night she met Jackson Hunt and they conceived Castle, during which, he recalls her playing "My Funny Valentine" on the piano. That song has been covered by, among others, Barbra Streisand, who is both a fan of Castle and the wife of Jackson Hunt's actor, James Brolin.
Dyeing for Your Art: In addition to the spoiler above, Natalie Rhodes used a complete makeover to "come closer" to Beckett's style, including using a wig to change her hair from blonde to brunette (neither of which happens to be Laura Prepon's natural hair color).
Enforced Method Acting: In the pilot, a scene was originally written where Beckett would question Captain Montgomery's wisdom in allowing Castle to be part of the investigation. When filming, the producers decided to remove this scene by having Montgomery bluntly shoot down Beckett's request to talk to him ("Nope.")... but didn't inform Stana Katic, whose look of annoyed astonishment is real.
Fake American: Both lead characters are played by Canadian actors.
Fake Brit: "The Limey", particularly the Scotland Yard detective who consults in the case, is very clearly an Australian.
Dana Delaney, who guest-stars in the two-parter "Tick...Tick...Tick...Boom!" is a multiple Emmy winner who was once a fixture on lists of the most beautiful people in Hollywood. Most tropers watching this show will probably recognize her as Lois Lane (leading to the always weird effect of Lois-sounding dialogue coming from the un-Lois-looking Delaney's mouth), or Katherine Mayfair, where Nathan Fillion played her husband.
Which gives a new layer of meaning to Nathan's line "Except for my second wife, this is the most sexless relationship I've been in."
"A Deadly Affair" has Wendy the waitress show up as the wife of one of the murder victims.
"Little Girl Lost" has Carla as the mother of the lost girl.
"3XK" has Drew from the last series of Scrubs as a suspect's terrified ex prison roommate who is in actuality the serial killer who masterminded the episode's killing in order to escape prison, as well as Marshall Darling as the father of a serial killer's cold-case victim.
"Always Buy Retail" has Hardison as a Nigerian immigrant.
Azazel appeared in Castle's supernatural-themed Halloween episode, where characters talked about ghosts and demons. Azazel himself finds the idea of demons to be ridiculous. He turned out to be the killer.
Lee Garner, Jr. is one of the people holding up a bank in "Cops & Robbers."
Arrow's John Diggle (David Ramsey) plays an elementary school teacher in season 2's "Fool Me Once".
Hey, It's That Place!: Sort of. The warehouse that they fight the murderer in "Boom!" in returns again in "Setup," with absolutely nothing to disguise it and several shots of the warehouse which are basically exactly the same in both episodes. Either they ran out of locations or the terrorist in setup used the same warehouse again. Lampshaded by Castle when he wonders why that particular building is so popular.
The hallway leading to the sleazy boyfriend's apartment in "Pretty Dead" seems very familiar - especially its distinctive square window.
They don't even try to hide the fact that the hangar they use in "Secret Santa" is the same hangar that was used for the climax of "Knockout".
Castle didn't actually go very far in his pursuit to retrieve Alexis in France. They use the same church in "Hunt" as the one previously used in "Always".
I Want You to Meet an Old Friend of Mine: Some of the people that help Castle and the 12th also happen to be people Nathan Fillion has worked with in the past, most notably Dana Delany in "Tick Tick Tick"/"Boom!" and Kristen Lehman in "Eye of the Beholder." Castle's first wife, Meredith, is played by Darby Stanchfield, who also played his wife in Waitress. Alyssa Milano is also apparently a friend of Nathan's, but the two haven't worked together except for "A Rose for Everafter." "Headhunters" finally brought a Firefly alumnus to the table in the form of Adam Baldwin. Gina Torres gets in the game in "Reality Star-Struck". Still waiting on the others! A minor one in Reality Star Struck (aka the one with Zoe), but notable as being between two guest stars, rather than a guest star and a main cast member: Patrick Fabian plays the reality show director, and Lachlan Buchanan plays the son of one of the show's stars. They previously worked together on the CMT sitcom Working Class.
Old Shame: In the pilot, one of the first things that clues Castle into the fact that things aren't what they seem with the copycat killings and gets him interested in solving the mystery is that the killer is staging them based off his self-described "lesser works".
One of Us: There is only one person who wants Castle and Beckett to get together more than the fans. That person is Stana Katic. Exhibit A. Exhibit B. 'Nuff said. On the other hand, Nathan Fillion emphatically did not want them to end up together, possibly fearing the Moonlighting curse of Shipping Bed Death.
Reality Subtext: Andrew Marlowe has suggested that the dynamic between Castle and Beckett — specifically, a writer and his 'muse' going on to develop a romantic bond with each other — mirrors his relationship with fellow writer Terri Miller.
Real-Life Relative: Seamus Dever (Ryan)'s wife Juliana has a recurring role as Jenny, Ryan's girlfriend and later, fiancée, and much later, wife.
The cast and crew have utilized it to very cute and very meta ends—such as creating a wedding website for Jenny and Ryan with the Devers' pictures.
Averted with Beckett's dad, who's just a regular character actor and even matches the photo shown in passing during the first-season.
Lyle Lovett as a member of The Men in Black in "Close Encounters of the Murderous Kind".
Adam Baldwin in "Headhunters". The ABC press release prior to the episode made gratuitous mention of his past in Firefly, the beloved but short-lived show with Nathan Fillion. Baldwin also did numerous interviews promoting his one-episode part before the show aired.
Another of Nathan Fillion's co-stars from Firefly, Gina Torres, appears in "Reality Star Struck".
Jackson Hunt in "Hunt", played by none other than James Brolin.
Castle's new main character, "Nikki Heat," is Beckett. To the point that when Beckett showed up at his launch event, the photographers said "It's Nikki!" In the actual Heat Wave, "Nikki" has Those Two Guys on her team: Raley and Ochoa, usually combined to "Roach." And she's being shadowed by a writer (this time, a reporter) who pulled strings with the mayor's office to get himself attached to her. Named Jameson Rook. Except Nikki and "Rook" had sex in the book — Castle has less patience than his show's writers, it seems.
In the episode "Boom", the FBI and NYPD teams find an autographed copy of Heat Wave that says, "To Scott: Write what you know." "What he knows" turns out to be murders. Lots and lots of murders. The same guy also turns out to be the killer.