* AbsenteeActor:
** Lanie (Tamala Jones) is absent in a few episodes, replaced by the grumpy male ME Pearlmutter.
** Notably, neither Lanie nor Perlmutter appear in the episode "Headhunters". Instead, there's a different coroner in the morgue, whom Detective Slaughter (played by Adam Baldwin) hits on.
** Both Lanie and Captain Montgomery (Ruben Santiago-Hudson) were missing in "Nikki Heat", presumably to make way for the big guest star (Laura Prepon).
** Captain Gates has been missing for swaths of episodes at a time in the 4th season. There's [[TyrantTakesTheHelm hating your boss]] so much you avoid her at every opportunity, and then there's ''this''. This also applies to a few episodes in season 5 as well.
** Interestingly, Lanie is nowhere to be seen in the season 5 premiere, the episode after [[spoiler:Beckett and Castle finally get together]]. Justified, though, as no homicide really occurs in the episode to necessitate her presence.
** Neither Alexis nor Martha appear in "Swan Song". Justified, in that the documentary crew get permission to follow the detectives at work as far as it pertains to the investigation of James Swan's murder, but presumably their home lives were out-of-bounds.
*** Alexis and Martha (and of course, their portrayers Molly Quinn and Susan Sullivan) have had several absences throughout the series (and probably have more non-appearances of any of the series' regulars, with Penny Johnson-Jerald's Captain Gates coming quite close); both actresses appeared in almost all of the episodes during the first season, with an increasing number of absences during the later seasons. Incidentally, this coincided with the beginning of the shift in storylines from a primary focus on Castle and Beckett's investigations and a secondary focus on Castle's family life to more of a focus on the former and less on the latter, a situation that became more prevalent from season five onward as Sullivan and Quinn mainly appear in only one or two brief scenes in the episodes they ''do'' appear. And even then, the two of them rarely appear together in the same episode, let alone the same scene, compared to earlier seasons.
*** The first half of season six was a little bit better about this in regards to Alexis. That part of the season featured a storyline in which Alexis began dating a guy named Pi,[[note]]Spelled like the number, not the dessert as Castle had surmised in one episode.[[/note]] whom Castle did not approve of and whom Alexis would briefly live with (first moving him into Castle's apartment and then moving in with Pi into their own apartment). This sparked a bit of a rift between the two, which would be resolved after the two teamed up to prove that a death row inmate charged in a murder did not commit said crime in "Like Father, Like Daughter", and later culminated in Alexis deciding to break up with Pi in "Limelight" and ultimately move back in with Castle and Martha.[[note]] The two aforementioned episodes were among the few, along with the season five two-parter "Target"/"Hunt", to involve Alexis as part of the main plotline and heavily feature her in the episode. [[/note]] Though the resolution of this storyline led to the return of the previous status quo regarding Alexis' appearances in subsequent episodes.
** "Cool Boys" (the second Slaughter episode) is the first episode in the series where Beckett doesn't appear.
* ActorAllusion: Like his actor, Jon Huertas, Esposito is a military veteran.
** The writers are excited to make {{shout out}}s to Nathan Fillion's ''Series/{{Firefly}}'' days and he has no problem obliging their requests, even adding in little ones himself when appropriate.
---> '''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 Series/{{Firefly}} (Wash) did. Notably, this is the episode guest-starring another Series/{{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/BattlestarGalactica1978 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? [[Series/{{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]]".
* CaliforniaDoubling: ''Castle'' is not filmed in New York, but Los Angeles. If you're someone who lives in Southern California (or knows the general L.A. area), then you can easily spot an orange Los Angeles Metro Bus just driving by in the background once in a while.
* CameoProp: The catalyzer from ''Series/{{Firefly}}'' can sometimes be seen on the set thanks to Nathan Fillion.
* TheCastShowoff:
** Stana Katic got to take on a Russian accent in "Deep in Death"; Russian is one of several Slavic languages she speaks in real life.
** Jon Huertas (Esposito) gets a chance to sing in "Swan Song". He's pretty good!
** As of "The Blue Butterfly," Tamal Jones (Lanie Parrish) shows off her lovely singing voice. (Said scenes are flashbacks imagined by Castle, although [[FridgeBrilliance if she's that good in his mind, he must have heard her before]].)
* CreatorBacklash: Castle's creator Andrew Marlowe, has discreetly stated before on Twitter that he is not fond of Season 8, and considers Season 7's "Hollander's Woods" the true series finale. He has also stated that had he known Season 8's "Crossfire" would be the actual final episode, he would have went ahead and written that episode himself, and the episode would have ended with an AndTheAdventureContinues moment, just as the Season 7 finale did.
* DawsonCasting:
** {{Averted|Trope}} by Alexis Castle as Molly Quinn is nearly the same age as her character.
** In "Suicide Squeeze", a 17-year-old was played by a 4-foot-10 30-year-old actress. The actress did not really look at all like a 17 year old... just like a short adult. [[spoiler:The writers probably realized this, because they played with it, making the victim's wife (and murderer) think the girl was the victim's lover when it was actually his daughter]].
* {{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.
* ExecutiveMeddling: Both Stana Katic (Becket) and Tamala Jones (Lanie) were not asked to come back for the proposed Season 9 in order to save money. However the point became moot when ABC decided not to renew the show. And it's stipulated in Fillion's contract that he only spends two days per week shooting with Katic. It's only in ''his'' contract, and he's a producer, so... Speaking of ''Moonlighting'', why is it that as soon as two leads get married in-universe that they invariably grow to hate each other's guts?
* 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.
* HeyItsThatPlace:
** Sort of. The warehouse where they fight the murderer in "Boom!" 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 [[Series/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". Creator/SummerGlau is added to the list in "The G.D.S." 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''.
* NamesTheSame: '''Richard Rodgers''': the name of one half of the famous duo of ''Creators/RodgersAndHammerstein'' or the birth name of crime novelist [[Series/{{Castle}} Rick Castle]]? Not entirely surprising since both Martha and her Nikki Heat counterpart Margaret Rook was nominated for a Tony Award...
* 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]]. 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 [[Series/{{Firefly}} Mal Reynolds]] [[http://www.themarysue.com/wp-content/uploads/2011/07/tumblr_lous31tXYd1qlb806o1_500-412x550.jpg during Comicon 2011.]]
* 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.
** Stana Katic's then-boyfriend (now husband) Kris Brljac appears as an extra on the season 5 episode "Death Gone Crazy" .
* RealLifeWritesThePlot: Apparently, the reason behind Beckett and Castle's relationship troubles in Season 8 was due to fighting between Stana Katic and Nathan Fillion.
* 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.
* RetroactiveRecognition: Maya Stojan was a supporting actress on ''Castle'' for quite some time (she has played Tori, the tech specialist, from midway through season 5 to the end of season 7), but it's likely most people haven't taken notice to her until after seeing her as Agent 33 on ''Series/AgentsOfSHIELD''.
* ScullyBox: Beckett's impractical-for-police-work [[CombatStilettos high heels]] help Stana Katic be in line with Nathan Fillion's height.
* SeriesFauxnale: The Season 7 finale "Hollander's Woods" doesn't end on a CliffHanger like most other season finales do. This is primarily because at the time of writing and filming, the show's possible Season 8 renewal was still up in the air. So, instead the episode ends with an AndTheAdventureContinues note (everyone leaves to investigate a new murder, after Castle receives a literature award). It's also interesting that the final scene is one of ''very'' few instances in which all the main cast members are present in the same scene (even Captain Gates is there!), as well as two supporting characters, Ryan's wife Jenny, and the tech cyber expert Tori. There was also some question as to if Stana Katic would return even if the show was renewed, thus the side plot of Kate questioning whether she should take an offer to [[spoiler: become a state senator]], which is clearly the writers setting up a PutOnABus situation. Thankfully, Katic signed on for a Season 8, once the show was officially renewed.
* 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 ''Series/DesperateHousewives'' Dana Delany as a federal agent. NathanFillion previously played Delany's husband on that show.
** "The Late Shaft" features Tom Bergeron, who just happens to host ''Series/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]].
* TheDanza: The background police officer "L.T", who has had some speaking parts throughout the series, is played by actor L.T. Tolliver.
** Pretty much any background character that Castle refers to by name. According to commentary, Nathan likes to ad-lib it whenever he can.
* TroubledProduction: In a vein similar to ''Series/{{Moonlighting}}'', Nathan & Stana couldn't stand working with each other, and would only do scenes in front of the camera. Whichever of the various stories of off-set behaviour is true or not remains a [[RuleOfCautiousEditingJudgement riddle for the ages]]. Most fans believe it only started getting bad after Season 4, because up through Season 3, the two of them were still doing joint interviews and promotional videos for the series: these abruptly stopped with Season 4.
* WhatCouldHaveBeen: Adam Baldwin stated at Dragon*Con 2016 that the writers wanted to use his character Slaughter in more episodes, but Baldwin's schedule conflicts with other shows (notably, ''TheLastShip'') prevented this from happening, and Slaughter only appeared in two episodes, one in Season 4, and once in Season 8.
* 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. Also, her black best friend ME Laney Parish becomes Nikki's black best friend ME Lauren Parry.
** 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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