* ActorAllusion: Like his actor, Jon Huertas, Esposito is a military veteran.
** The writers are excited to make {{shout out}}s 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/{{Castle}} 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 [[http://www.youtube.com/watch?v=cwQ5PbuKdoE 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 [[OneLifeToLive One Life To Live]]". Nathan Fillion's first TV role was [[SoapOperaRapidAgingSyndrome 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 [[Film/TheMummy1999 mummy's curse]].
** In "Close Encounters of the Murderous Kind," Castle shows a proficient knowledge of Chinese and mentions he learned it [[Series/{{Firefly}} "from a TV show [he] used to love]]."
** "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 Name}}s 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? "[[Series/{{Firefly}} Shiny]]."
*** Castle disparages the fictitious canceled television show Nebula 9, saying "Really? How is Nebula 9 worth all this? I mean, [[Series/{{Firefly}} it was canceled over a decade ago after twelve episodes!"]]
*** Castle mentions he's a fan of ''good'' sci-fi shows, like Franchise/StarTrek, [[Series/BattleStarGalacticaClassic Battlestar,]] and "[[Series/{{Firefly}} that Joss Whedon show]]."
*** 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 "[[Series/{{Firefly}} Heart of Gold]]" can be seen on the table of Benjamin Donnelly, the SciFi tech dealer Castle and Beckett encounter (who, by the way, was played by Armin Shimerman, AKA [[Series/StarTrekDeepSpaceNine 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 ''{{Series/Firefly}}''.
** Played with somewhat in "Reality Star Struck" which guest-stars Gina Torres, who played Zoe in ''Series/{{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 [[Film/FantasticFour 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 [[Series/TheLyingGame 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.
** Actor allusions even make their way into the [[{{Defictionalization}} real-life Nikki Heat books]]:
*** 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 [[WesternAnimation/{{TheSimpsons}} Homer Simpson]]. In the book, Jameson Rook's judge friend is named... Horace Simpson. Nikki even expects him to say, [[{{Catchphrase}} "D'oh!"]] when she beats him at a poker game hosted by Rook.
*** In ''Naked Heat'', Detective Heat sends two cops off on an assignment. Their names? [[{{Firefly}} Malcolm and Reynolds.]]
*** One of the suspects in ''Heat Rises'', the third book, mentions as his alibi that he was at home, because he had rented ''Film/QuantumOfSolace'', 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 ''Film/QuantumOfSolace'' [[spoiler: 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, Main/BarbraStreisand, who is both a fan of ''Castle'' and the wife of Jackson Hunt's actor, James Brolin.
** In "Room 147", Castle mentions how he likes "[[WebVideo/DoctorHorriblesSingAlongBlog the weird stuff]]".
* CameoProp: The catalyzer from ''{{Series/Firefly}}'' can sometimes be seen on the set thanks to Nathan Fillion.
* DawsonCasting: Averted by Alexis Castle as Molly Quinn is nearly the same age as her character.
* {{Defictionalization}}:
** ''[[http://abc.go.com/shows/castle/heat-wave Heat Wave]]'', the first Nikki Heat novel made it onto the Times bestseller list as did ''[[http://abc.go.com/shows/castle/naked-heat Naked Heat]]'' and ''[[http://abc.go.com/shows/castle/heat-rises Heat Rises]]''.
** ''[[http://www.richardcastle.net/news/storm-graphic-novel Deadly Storm]]'' is a graphic novel "adaptation" of the first Derrick Storm novel. ''Storm Rises'' has also been announced.
** ''[[http://www.amazon.com/Brewing-Storm-Derrick-Short-ebook/dp/B007A577SQ/ref=sr_1_1?s=books&ie=UTF8&qid=1342413485&sr=1-1&keywords=a+brewing+storm A Brewing Storm]]'' and ''[[http://www.amazon.com/Raging-Storm-Derrick-Short-ebook/dp/B007AE2RDW/ref=sr_1_2?s=books&ie=UTF8&qid=1342417116&sr=1-2&keywords=a+brewing+storm A Raging Storm]]'', two Derek Storm shorts.
* DyeingForYourArt: 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).
* EnforcedMethodActing: 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.
* FakeAmerican: Both lead characters are played by Canadian actors.
* FakeBrit: "The Limey", particularly the Scotland Yard detective who consults in the case, is very clearly an Australian.
* FandomNod: "One Life to Lose" is partially about the rabid shipping community surrounding a fictional soap opera. Many a PortmanteauCoupleName is used.
* HeyItsThatGuy: Overlaps frequently with [[YMMV/{{Castle}} Narrowed It Down To The Guy I Recognize]]. If you don't see a person here, chances are they're there, as one of the suspects of the week.
** NathanFillion is Castle.
** Stana Katic ([[Series/{{Heroes}} Hana Gitelman]], [[VideoGame/BatmanArkhamCity Talia al Ghul]]) is Beckett.
** And [[Series/BattlestarGalacticaReimagined Samuel Anders]] is a RomanticFalseLead.
** Season 2's "The Fifth Bullet" has [[BoyMeetsWorld Shawn]] as one of the suspects being questioned by Esposito.
** [[Series/BuffyTheVampireSlayer Riley]] featured in a surprising twist and a relationship with [[Series/{{House}} Cut Throat Bitch]].
** [[{{Warehouse13}} Myka]] is also a writer in a Season 1 episode.
** [[Series/{{Alphas}} Kat]] probably doesn't remember the month when she wrote webisodes for a sci-fi series.
** [[Series/{{Eureka}} Nathan Stark]] apparently didn't die in Eureka; he became a sci-fi actor.
*** His ex-wife is Gates's sister.
** Soap fans already had [[OneLifeToLive Joey Buchanan]] and [[Series/GeneralHospital Ian Devlin]] in the main cast; they threw in Carly/Claudia as Castle's date and even Winnifred for a scene in "The Third Man".
** Looks like [[Series/StarTrekVoyager The Doctor]] finally settled on his name: Dr. Death!
** Looks like [[Series/TwentyFour Vladimir Bierko]] gave up on the terrorist gig and became a fashion designer, of all things.
** Whedon fans have run into several other familiar faces: [[Series/BuffyTheVampireSlayer Principal Wood]] as a CorruptCorporateExecutive, [[Series/BuffyTheVampireSlayer Riley Finn]] as an amnesiac witness, [[{{Dollhouse}} Dominic]], [[Series/BuffyTheVampireSlayer Forrest]], and [[Series/BuffyTheVampireSlayer Principal Flutie]] as the precinct's lawyer in the second episode. We also have Creator/AdamBaldwin as Detective Slaughter.
** 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 [[TheDCAU Lois Lane]] (leading to the always weird effect of Lois-sounding dialogue coming from the un-Lois-looking Delaney's mouth), or [[DesperateHousewives Katherine Mayfair]], where [[IWantYouToMeetAnOldFriendOfMine 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."
*** The same two-parter has [[VideoGame/{{Borderlands2}} Handsome]] [[DameonClarke Jack]] as the villain.
** One recurring [=HITG=] is PhilLaMarr.
** Creator/DanCastellaneta has a recurring role as a judge.
** [[Film/TheMummy1999 Erick Avari]] would like to warn you about Mayan Curses. A fantastic CastingGag, especially if you recall how he kicked it in the movie.
** Theresa from Series/TheOC (or Amita from ''Series/{{Numb3rs}}'') has apparently got a degree in Archeology and is working with mummies.
** "Den of Thieves" has [[GilmoreGirls Max Medina]] and [[Series/{{Alias}} Francie]].
** MichaelIronside is a suspect of the week.
** [[Series/TheXFiles Assistant Director Walter Skinner]] (or [[Series/StargateAtlantis Colonel Caldwell]]) shows up in "A Deadly Game" as a hardened spy-stroke-assassin [[spoiler: who ends up turning out to be some hapless schmuck on a spy vacation.]]
** "A Deadly Affair" has [[Series/HowIMetYourMother Wendy the waitress]] show up as the wife of one of the murder victims.
** "Little Girl Lost" has [[Series/{{Scrubs}} Carla]] as the mother of the lost girl.
** "3XK" has Drew from the last series of Series/{{Scrubs}} as a suspect's terrified ex prison roommate [[spoiler:who is in actuality the serial killer who masterminded the episode's killing in order to escape prison]], as well as [[Series/ClarissaExplainsItAll Marshall Darling]] as the father of a serial killer's cold-case victim.
** "Always Buy Retail" has [[Series/{{Leverage}} Hardison]] as a Nigerian immigrant.
** "One Life To Lose" has [[{{Psych}} Henry Spencer]] as a soap opera actor.
** "Under the Gun" has [[Series/{{Charmed}} Leo Wyatt]] as a priest who was also a suspect, [[Series/{{Shark}} Raina]] also known as [[{{Numb3rs}} Nikki]] as the victim's wife and [[MonkeyShines Jason]] as Beckett's mentor.
** "The Mistress Always Spanks Twice" has [[Series/{{Supernatural}} young Mary Winchester]] and [[{{Saw}} Allison Kerry.]]
** "He's Dead, She's Dead" features [[Series/AmericanDreams Beth Mason]] as the daughter and [[MySoCalledLife Patty Chase]] as the widow of the victim.
** Two ''Series/{{Reaper}}'' alumni, Ray Wise and Rick Gonzalez, have appeared on the show as a baseball player's agent and a squatter, respectively.
** Creator/LanceHenriksen is an abduction therapist in "Strange Encounters of the Murderous Kind"
** "Nikki Heat" has [[That70sShow Donna Pinciotti]] as the actress (in-universe) who plays Nikki Heat on screen.
** In "Poof! You're Dead", [[StickIt Joanne]] helps with the magician's investigation. [[Film/TheReplacements Martel]] is the real murderer, it turns out.
** [[Series/{{Bones}} Dr. Goodman's]] five-season mystery sabbatical is finally explained--he becomes a drug lord who can very easily get under the skin of Castle and especially Beckett.
** As of 'Set Up', [[Series/{{Heroes}} Nathan Petrelli]] works for the actual Department of Homeland Security in this universe. Trying to [[spoiler: prevent a nuclear bomb from going off]] in New York City. Sure it's not Peter?
*** And Noah Bennet is the corrupt Senator Bracken.
** [[Series/{{Jericho}} Mimi]] is an interior designer.
** [[Series/TheGoodGuys Lt. Ruiz]] is in the 12th precinct, seen in "When the Bough Breaks" and "The Double Down".
** In "Law & Murder", one of the jurors has appeared before in the show.
** [[Series/VeronicaMars Mac]] is an intern on a soap opera in "One Life to Lose".
** [[Series/LieToMe Loker]] is an up-and-coming writer that Castle mentors in "Dead Pool".
** [[TheWestWing Helen Santos]] is now married to a Donald Trumpalike?
** Said Donald Trumpalike is [[Film/ThisIsSpinalTap David St. Hubbins]].
** [[Series/{{NCIS}} Mike Franks]] is an antique gun nut in "Punked".
** [[Series/EvenStevens Steve Stevens]] is the presiding judge in the trial in "Law & Murder."
** [[TwoThousandOne Dave Bowman]] appears in the pilot episode as the father of one of the victims.
** Alexis' boyfriend is Ben from ''TheSecretLifeOfTheAmericanTeenager''.
** MichaelDorn, who starred as Worf in ''Series/StarTrekTheNextGeneration'' and ''[[Series/StarTrekDeepSpaceNine Deep Space Nine]]'', is Beckett's police-mandated therapist.
** Our new captain was [[Series/StarTrekDeepSpaceNine Kasidy Yates]].
** [[Series/{{Supernatural}} 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. [[spoiler:He turned out to be the killer.]]
** [[MadMen Lee Garner, Jr.]] is one of the people holding up a bank in "Cops & Robbers."
** [[Series/{{Lost}} Pierre Chang]] is now leading a Chinese gang.
** It seems ''Castle'' has become something of a ''Series/StarTrekDeepSpaceNine'' Reunion Party, because [[Creator/NanaVisitor Major Kira]] [[Series/StarTrekDeepSpaceNine Nerys]] featured in "An Embarrassment of Bitches".
*** Which also featured [[Series/{{Smallville}} Green Arrow]].
** Also prominently featured in "An Embarrassment of Bitches"? Hilarie Burton, known to some as [[OneTreeHill Peyton Sawyer]] and [[WhiteCollar Sara Ellis]].
** Beckett's mysterious benefactor holding the attack dogs at bay seems to have stepped up from his [[UnhappilyEverAfter previous miserable life]].
** [[Franchise/MassEffect Garrus]] is found chained up in his apartment in one episode.
** [[{{Dollhouse}} Paul Ballard]] is the one who tried to kill Beckett. The Dollhouse must be involved in the conspiracy.
** Creator/DCDouglas shows up in "Dial M for Mayor" as a lawyer.
** [[Series/TeenWolf Derek Hale]] appears in "Fool Me Once" as Alexis's new violin teacher. Teen Wolf fans are shocked to discover that he's capable of smiling.
** [[{{TrueBlood}} Nan Flanagan]] is an art collector.[[spoiler: And murdered a curator.]]
** [[{{Series/BattlestarGalacticaReimagined}} Helo]] is Cole maddox, in a shocking contrast to his earlier role.
** [[Series/GreysAnatomy Sarah Drew]] in "Nanny [=McDead=]".
** [[{{PrisonBreak}} Gretchen]] now works in television, and even tries to get Castle into bed with her.
** Apparently, [[Series/{{Lost}} Phil]] has a habit of [[spoiler: backstabbing main characters]].
** [[Series/StargateAtlantis Todd the Wraith]]/[[Series/{{Sanctuary}} Druitt/the Big Guy]] is a French black-ops freelancer in "Hunt".
** [[Series/OnceUponATime Jiminy Cricket]] is Dr. Daryl Meeks, the Bigfoot "expert."
** The show also has something of an all-star ''crew'', especially if you were a sci-fi fan in TheNineties and TheNoughties (which may explain some of the guest stars):
*** Rob Bowman - who directed 33 episodes of ''Series/TheXFiles'' and the first movie (''Film/TheXFilesFightTheFuture'') is a featured director and ExecutiveProducer, and cut his teeth directing ''Series/StarTrekTheNextGeneration'', which allowed him to pull in Creator/JonathanFrakes to direct a few episodes (most notably "The Final Frontier")
*** Thomas J Wright - who directed a full third of ''Millenium'' also pops up occasionally.
*** Rene Echevarria - of ''Star Trek TNG'' & ''[=DS9=]'' fame - wrote for the first two seasons.
*** Jose Molina (''Firefly'') is another recurring writer.
*** As is David Amann (''The X Files'').
*** Even the composer fits here: It's Robert Duncan, who worked on the later seasons of ''Series/{{Buffy|The Vampire Slayer}}''.
** [[{{Film/FantasticFour}} Mr. Fantastic]] is billionaire Eric Vaughn.
** "A Rose for Everafter" has [[Series/{{Charmed}} Phoebe Halliwell]] as Castle's old flame.
** James Brolin is [[spoiler:Castle's father]].
** [[Franchise/MassEffect Kaidan Alenko]] is a Bigfoot hunter.
** The director of the ''Heat Wave'' movie is [[WesternAnimation/TheLegendOfKorra Bolin]].
** Meet Beckett's new partner: [[Series/{{House}} Cuddy]].
** [[Series/{{Alphas}} Hicks]] is apparently a journalist in DC.
** "Time Will Tell" has [[Series/{{Chuck}} Joshua Gomez]] and [[Series/StarTrekVoyager Tim Russ]].
** ''Series/{{Arrow}}'''s John Diggle (David Ramsey) plays an elementary school teacher in season 2's "Fool Me Once".
** Molly, the girl in the Irish pub who wanted out in "Sucker Punch", was [[Theatre/{{Wicked}} principal Nessarose and understudy Elphaba.]]
* HeyItsThatPlace: 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 [[spoiler: 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 [[spoiler:pursuit to retrieve Alexis]] in France. They use the same church in "Hunt" as the one previously used in "Always".
* IWantYouToMeetAnOldFriendOfMine: Some of the people that help Castle and the 12th also happen to be people NathanFillion has worked with in the past, most notably [[DesperateHousewives Dana Delany]] in "Tick Tick Tick"/"Boom!" and [[Series/{{Drive}} Kristen Lehman]] in "Eye of the Beholder." Castle's first wife, Meredith, is played by Darby Stanchfield, who also played his wife in Film/{{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 Series/{{Firefly}} alumnus to the table in the form of Creator/AdamBaldwin. GinaTorres gets in the game in "Reality Star-Struck". Still waiting on the others! A minor one in Reality Star Struck (aka the one with [[Series/{{Firefly}} 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 ''Series/WorkingClass''.
* OldShame: 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".
* OneOfUs: There is only one person who wants Castle and Beckett to get together more than the fans. That person is Stana Katic. Exhibit [[http://youtu.be/DWqXABPC2xU A]]. Exhibit [[http://youtu.be/y_afnu8Xx20 B.]] 'Nuff said. On the other hand, Nathan Fillion emphatically did '''not''' want them to end up together, possibly fearing the ''Moonlighting'' curse of ShippingBedDeath.
** Molly Quinn is a real-life cosplayer, famously playing a female [[{{Firefly}} Mal Reynolds]] [[http://www.themarysue.com/wp-content/uploads/2011/07/tumblr_lous31tXYd1qlb806o1_500-412x550.jpg during Comicon 2011.]]
* RealitySubtext: 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.
** In one episode, Ryan is pressured into eating health food by his wife, and it has a bad effect on his work and health. In reality, both Seamus Dever and his wife, who plays his wife on the show, are already vegetarians.
* ScullyBox: Beckett's impractical-for-police-work [[CombatStilettos high heels]] are this for Stana Katic.
* RealLifeRelative: Seamus Dever (Ryan)'s wife Juliana has a recurring role as Jenny, Ryan's girlfriend [[spoiler: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.
* RetroactiveRecognition: On a big-name show with 6 seasons under its belt, probably bound to happen, too. Check under HeyItsThatGuy, though.
* StuntCasting:
** From "A Rose for Everafter", NewOldFlame Kyra Blaine, played by Alyssa Milano. Oh, and Sophia Turner, played by Jennifer Beal in the season 4 two-parter "Pandora"/"Linchpin".
** The two-episode arc, "Tick, Tick, Tick" and "Boom" has ''DesperateHousewives'' Dana Delaney as a federal agent. NathanFillion previously played Delaney's husband on that show.
** "The Late Shaft" features Tom Bergeron, who just happens to host ''DancingWithTheStars'', ''Castle'''s lead-in.
** 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 TheMenInBlack 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 ''{{Series/Firefly}}'', Gina Torres, appears in "Reality Star Struck".
** Jackson Hunt in "Hunt", played by none other than [[spoiler:James Brolin]].
* WriteWhoYouKnow:
** 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 ThoseTwoGuys 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 [[spoiler: murders. Lots and lots of murders.]] The same guy [[spoiler:also turns out to be the killer.]]
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