* ActingForTwo:
** Masaki Suda appears in flashbacks as [[spoiler:young Katsumi Daido]] during ''Forever A to Z'' and [[spoiler:Eternal's V Cinema]]. Though this may simply be [[spoiler: [[FridgeBrilliance Maria having all but forgotten what young Katsumi was like, and inadvertently imprinting Phillip as a]] SuspiciouslySimilarSubstitute]].
** Hikaru Yamamoto plays Melissa in the Double part of ''Movie War Core''. [[LampshadeHanging Akiko even notes the resemblance]].
* ActorAllusion: Aya Sugimoto, best known as [[Series/PrettyGuardianSailorMoon Queen Beryl]], appears in TheMovie ''Double Forever: A to Z'' as a Maria S Cran'''berry'''.
** Shotaro threw an epic one in "J's Labyrinth" when he wants to enter a models-only recreation club - Renn Kiriyama is, in reality, an actor/model.
---> '''Guard''': Members only, please. This club is for models only.
---> '''Shotaro''': Haha, that's sweet. ''I'm'' a model. So, ciao.[[note]]After which he tries to enter and immediately pushed back by the guard.[[/note]]
** Also in ''Double Forever'', Elizabeth manages to recover the T2 '''A'''ccel, '''K'''ey, and '''B'''ird Memories, a nod to the fact that her and Queen's actresses are members of the girl group [=AKB48=].
** Episode 45's first two minutes are devoted to reminding us of [[Series/JukenSentaiGekiranger Yuka Hirata's]] previous role by letting her perform her [[{{Tsundere}} extreme tsun-tsun, extreme dere-dere]] schtick.
** Yoshie Gotou, one of the Stage Moms in "The O Chain", actually appeared in Series/KamenRiderKiva as the woman who went into sports archery after her own StageMom pushed her to learn the violin. Karma's the real bitch here.
** At one point, Jinno notes that Takeshi would be a nice name. Also, his use of GratuitousEnglish mirrors his schtick as a FakeAmerican. And the name "Catherine" pops up later... [[note]]Nadagi is part of the duo "Dylan & Catherine"[[/note]]
** Masayuki Deai (Trigger Dopant), though not for one of his own characters. In ''W Returns: Eternal'', he gives the classic [[Series/GoGoSentaiBoukenger BoukenRed]] "[[GratuitousEnglish Good job !]]" and thumbs up.
** In ''Begins Night'' (W's half of ''Movie Wars 2010'') there's this hefty Museum {{Mook}} (GiantMook?) who jumps and attempts a Rider Punch on Soukichi. He's played by Jiro Okamoto, suit actor for KamenRiderBlack.
* [[AuthorExistenceFailure Actor Existence Failure]]: Minoru Tanaka committed suicide after the film was released.
* CastingGag: The boy who Shotaro helps (and teaches the way of being hard-boiled) in the last episode is played by the same actor who played preteen Shotaro in the first episode flashback. He also plays Shotaro again in a small part of Movie War Core in the Skull section.
* DawsonCasting: Mitsuru Matsuoka was ''40'' when he played Katsumi, who appears to be in his early to mid-20s based off the information given. Possibly justified since storywise he was [[spoiler:growing up using an artificial chemical.]]
* {{Defictionalization}}: Queen and Elizabeth are played by two members of idol group [=AKB48=], and when they released their joint single "Love♡Wars", the Liar Dopant arc had them appear on an American Idol-esque contest (Fuuuuuutic Idol) performing the song in order to win a major label release.
* ExecutiveMeddling: {{Averted}}; the show's staff said that there was incredible pressure from Toei to make Double's primary color red (which is viewed as a heroic color in Japan, and is the reason the LawOfChromaticSuperiority applies not only to [[Series/KamenRiderKuuga several]] [[Series/KamenRiderRyuki previous]] [[Series/KamenRiderKabuto Kamen]] [[Series/KamenRiderDenO Riders]] but to Franchise/SuperSentai as well). They apparently had to fight long and hard to avoid this, and only managed to get black and green as Double's default form by citing [[Series/KamenRider Riders 1 and 2]] as their inspiration.
* FakeNationality: Kazu is played by Korean actor Gong Teyu.
* FanNickname: Shirihiko [[note]] ''Shiri'' means butt[[/note]] and N'''ass'''ca, for his [[{{Fanservice}} butt scenes]].
** Isaka is "Weatherman".
** Mick is the [[WebOriginal/LOLCats Cheezburger Dopant]]
* NamesTheSame: There's a Nasca in the third KeroroGunsou movie. [[spoiler:She's ''also'' an alterego (real name Miruru), with the ability to sprout wings]].
** Was the Taboo Dopant inspired by the BlackEyedPeas?
*** A pity then, that the W isn't for will.i.am...
** And yeah. Saeko Sonozaki. Did she share a name with a certain ''Series/KamenRiderFaiz'' character and had a similar position for the villains?
*** [[VisualNovel/HigurashiWhenTheyCry Sonozaki]] you say? As an added bonus, Wakana was in a live-action adaptation thereof as Mion.
** Sokichi Narumi, with the white hat and all, brings to mind a certain Sokichi Banba, better known as Big One of ''Series/JAKQDengekitai''.
** One guest in "The F Afterglow" is an ex-thief named Fuyumi Aso, which means she may have relatives [[Series/KamenRiderKiva in another parallel world]].
*** Make that [[Film/KamenRiderZO TWO parallel worlds]].
** 'Fangs' are also tiny self-mobile weapons used by [[Anime/MobileSuitGundam00 Gundam Throne Zwei and Arche Gundam]].
** There also was an Isaka in ''Series/KamenRiderBlade''.
** From "The B Carried on the Wind", Isamu Bitou, much like [[Series/MahouSentaiMagiranger Isamu Ozu]], had been locked away from society for some time (albeit for different reasons). [[spoiler:It's worth noting that [=CycloneJokerXtreme=], who debuts here, carries a sword and shield that slide together, much like Wolzard's did]]. He also has the same family name as [[Series/NinpuuSentaiHurricaneger Kouta Bitou]], [[TwoGuysAndAGirl who also is friends with a guy and a girl,]] though in this case all three are JustFriends (close friends, but still).
** The client from episodes 45-46 is named Kyoko [[Series/KamenRiderHibiki Todoroki]].
** Surprisingly, Shotaro, GenreSavvy that he is, didn't freak out at the fact that his partner's true name is [[spoiler: [[Manga/DeathNote Raito]] [[VisualNovel/HigurashiWhenTheyCry Sonozaki]]]].
** The Narumi Detective Agency has absolutely nothing to do with [[RaidouKuzunohaVsTheSoullessArmy a certain other one that deals with a certain soulless army.]]
** The client in episode 49 wishes to see his sister [[Series/KamenRiderRyuki Yui]] rescued. They're both someone's sister and in need of rescue constantly, but then again that's kind of a staple female role on Franchise/KamenRider.
** A red warrior named Ryu whose motivation is the loss of a loved one and who gets the girl at the end? [[Series/ChoujinSentaiJetman Not the first time Toei's done it.]]
* PromotedFanboy: Renn Kiriyama was a fan of Series/KamenRiderBlack as a child.
* WhatCouldHaveBeen: [[spoiler:"The O Chain" storyline has Shroud mention [=CycloneAccel=] Xtreme, and we even get an ImagineSpot-style image of it, but it doesn't happen and probably never will because it would involve breaking up Shotaro and Philip's partnership as well as fighting with hatred rather than compassion, two things Ryu refuses to do.]]
** [[spoiler: Though we did get [[FanNickname CAX]] in Kamen Rider Climax Heroes [[Series/KamenRiderOOO OOO]].]]
* WordOfGod: The series itself is confirmed by the staff to be a homage to the Showa Era, from the little things such as the scarf to the overall feel of the series.
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