* ActorAllusion: Like his actor, Jon Huertas, Esposito is a military veteran.
* DawsonCasting: Averted by Alexis Castle as Molly Quinn is nearly the same age as her character.
* 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.
* 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 [[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".
* 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''.
* RealLifeRelative: Kevin Ryan's (Seamus Dever) girlfriend/fiancée/wife, Jenny O'Malley, is played, when she appears, by Dever's real-life wife Juliana.
* RetroactiveRecognition: On a big-name show with 6 seasons under its belt, probably bound to happen, too. Check under HeyItsThatGuy, though.
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