Fanon is an odd thing. Fans come up with it for many reasons. Because Word of Dante says so, because Word of God never said so, because it's the only plausible explanation of an event (sometimes it even is!), or just because it's fun!
One popular form of fanon, however, is that which is created simply to make shipping the OTP easier. This is the fanon referred to as The Ship's Motor. In the way that having a motor is helpful to a literal ship, this piece of fanon is helpful to a Fan Fic writer's ship. But it's not enough for it to be just helpful, to truly be The Ship's Motor, a piece of fanon must have no other clear purpose than to make shipping two characters more plausible. That does not mean the piece of fanon has to be contrived, or even implausible. It can be neutral, or even plausible in canon. Plausible or not, many of these become well-regarded in the fandom because so many fans ship the pairing.
Only tangentially related to the Youtube vidcast The Ship's Closet.
- Some Naruto fanfics with the Neji×Hinata couple have the two arranged to marry in order to keep the Byakugan bloodline trait within the family, thus precluding any other ships.
- Many Neon Genesis Evangelion fanfics with the Shinji×Rei pairing will bring up various arguments and bits of fanon that make the pairing less problematic by reducing the degree of genetic connection between the two.
- Death Note: During the time period when L and Light were handcuffed together, it is not mentioned how they did things such as sleep, bathe etc. It's quite possible (and likely) that they were unhandcuffed and someone else kept an eye on Light- but slash fic writers have decided they definitely slept in the same bed and bathed together.
- In The Force Awakens fandom, the idea that Rey and Finn are both sexually and romantically inexperienced due to their unusual childhood is popular among fans who ship Rey×Finn×Poe. This allows Poe to be introduced to an established Rey×Finn relationship in the process of "teaching" them how to kiss and so on. It also has some support through Word of Saint Paul, with John Boyega (who plays Finn) stating that he believes Finn is a virgin.
- Harry Potter:
- The Head Boy and Head Girl share quarters. Which seems particularly odd since the Founders use magic to keep boys from even visiting the girls' dormitories, but presumably the reasoning is that anyone who earns that title probably has enough sense to avoid unfortunate mishaps. Some of the saner examples downplay the trope a bit and give them separate sleeping quarters but a shared study/living room.
- There's a million of these, really. There are also "Marriage Law fics," where the Ministry passes a law forcing purebloods and Muggle-borns to marry against their will (usually for the sake of Snape×Hermione or Draco×Hermione). This... worked better before the last couple of books came out (not that was particularly plausible then). Another plethora of stories has Hermione wind up really being a Pureblood so that she more easily hook up with Malfoy or Zabini. Changing a character's heritage to pair her with an unrepentant Politically Incorrect Villain? Unfortunate Implications be danged!
- Star Trek: The Original Series:
- Kirk can empathically sense when Spock is going into Pon Farr. This actually crops up in one of the published TOS books, "Killing Time", which was famously recalled and edited after the publishers realized the author was a Kirk×Spock shipper.
- Also please note that Gene Roddenberry's novelisation of Star Trek: The Motion Picture stated that it was Kirk's mind that called to Spock across the light years and ended his chances at the Kolinahr. Maybe not such a stretch after all...
- Similar to the Harry Potter example above, a significant number of fics feature the idea that the Captain and First Officer have adjacent quarters and share a bathroom. Canonically their quarters appear to be on different decks altogether, but this piece of Fanon is so widely used (and convenient) that its use can be considered an Acceptable Break From Canon.
- In the Firefly fandom, River/Jayne shippers often use the idea that Jayne's thoughts are "clear" or "simple" as part of the reasons River might be attracted to him.
- Kara and Lee of Battlestar Galactica are believed to have gone to flight academy together even though canon has discredited this.
- Faith of Buffy the Vampire Slayer is often portrayed in fanfic as bisexual, building on her on-screen Anything That Moves nature. This is, of course, useful for pairing her with Buffy, Willow, or any other female character. This attitude of making a character bisexual can also extend to Buffy herself when pairing her up with Willow in earlier seasons or Faith or Tara in later seasons.
- The X-Files fans had a setup for Mulder and Scully's hotel arrangements - adjoining rooms, door in the middle connecting them — ostensibly so they could protect each other better from one their Monsters of the Week deciding to attack.
- It became fanon for Brooke/Sam shippers that they had an adjoined bathroom connecting their bedrooms, even though there was no such thing.
- Sam×Freddie belligerent sexual tension shippers use a reply that Word of God left on his blog as their motor, it's along the lines of "their bickering is a game, and maybe they are afraid to stop playing". The reasonable idea is that any 'game' is their inability admit they really care for each other as friends. The Sam/Freddie shippers take it as the 'game' is wilfully ignoring romantic feelings for each other because they enjoy fighting, and one-day will stop playing and get together.
- Sam being bi is a big piece of fanon that's very useful in bringing out Carly's If It's You, It's Okay feelings for her in the Carly/Sam fandom. The other common fanon is to make Carly and Sam both bisexual to make their mutual attraction easier or help ship Carly with Shelby Marx or Missy.
- Power Rangers S.P.D.: Sky being Bridge's roommate because his hardnosed-ness means his thoughts are more ordered and less apt to bleed over and give Bridge a psychic headache. (Actually, Sky's as emotional as anyone else, if not more. He just feels the need to act like the Perfect Officerï¿½ 24-7 because he wants to follow in the foosteps of his legendary pa.)
- Supernatural: Despite the prevailing opinion of the fandom, Sam and Dean having amazingly bright/pure souls—despite everything they've been through—has never actually been stated in canon. Their various angel lovers will be so disappointed...
- The Portal fandom has a habit of taking bots like GLaDOS and Wheatley, and making them humanoid, either by downloading them into a human body, giving them some sort of android, or finding a way to mix the two. This is done in part to make writing their expressions easier in fanfiction, but mainly it seems to pair them with Chell. This was only reinforced after Portal 2 revealed GLaDOS herself used to be human. Wheatley being the more human of the two (with less mechanical feedback in his speech, truly incredible fallibility, and "facial" gestures,) expect his androids and cyborgs to appear more natural, while GLaDOS's will have wires or damage right out in the open.
- Like the Buffy example above, Cody on Total Drama is often made bisexual, because while it's very clear that Cody likes girls, most fans prefer a bit of Ho Yay between him and Noah.
- Invader Zim:
- The fandom will often justify sex with Irkens by claiming that Irkens have sex at all. Canon seems to imply that they're all grown in a big factory, and naked Irkens have no apparent genitals. It's particularly common to assume that they lay eggs.
- Also, ZATR fans often portray Zim and Tak having known each other prior to the events on Devastis, even to the point of having been best friends since they were smeets. From the way Zim acts when Tak reveals herself on Earth it seems they never met before.
- Though Dib/Gaz isn't very popular, those who do ship it sometimes use a Word of God statement about Dib being an experiment created by Membrane to suggest that he and Gaz aren't really related. The creator notably never mentioned how Gaz fits into that scenario; since Fanon often makes Dib Membrane's clone, some extend this theory to make Gaz a clone of Membrane's dead wife in a weirdly literal case of Generation Xerox.