Crossovers can be a really fun kind of fiction to write; there's something really appealing in imagining fictional characters from numerous different properties interact in one story; how would they interact? Would they gel or collide?

Of course, like any story type, there are plenty of crossovers out there that suck. But luckily, you have the helpful folks here at TV Tropes to help steer you right.

Remember, check out SoYouWantTo/WriteAStory for writing advice that transcends genre, in order to make your story the best it can possibly be. See also SoYouWantTo/WriteAFanfic, for advice on writing in the medium where most crossovers ultimately take place.

!'''Necessary Tropes'''
What we're looking at here is a {{Crossover}}, which implies two or more fictional characters from different shows / media properties interacting within one show where normally they wouldn't. The different types of Crossover that you can choose from can be found on the CrossoverIndex, but for an official work, this generally involves two shows which are agreed or established to take place within the same fictional universe -- and which, naturally, the producers hold or can easily access the rights to. For unofficial fan-written crossovers, since the whole thing is unofficial anyway then the sky's the limit; you can bring together whoever you want.

LetsYouAndHimFight is also quite common; it seems to be a matter of course that the first thing that heroes from one property will do is lay into the heroes from the other property automatically and as a matter of course.
VillainTeamUp is just as common; often after the misunderstanding that led to the heroes' fight is solved, they will join forces to battle a villain from each of their franchises, who just so happen to be working together at the time.

!'''Choices, Choices'''
Who are the fictional characters you are bringing together? What universes are they from? How are they coming together? A lot of these questions can be answered by looking at the CrossoverIndex, but there are basically two types of crossovers.

* The first type is the Dimensional Crossover: Each series involved is in their own dimension, with some kind of travel (accidental or intentional) or NegativeSpaceWedgie bringing them together. In these cases, a good portion of the story is usually involving how to get the character(s) who are out of time and out of place back to where they belong. '''Example''': [[Creator/MarvelComics Marvel]] and [[Creator/DCComics DC]]'s ''[[ComicBook/JLAAvengers JLA/Avengers]]'' crossover was done like this.
* The other type of crossover is when the two series are implied to have always co-existed, but distance, focus, or circumstances have kept the two apart, until now. These crossovers leave the writer open to bypass the mess of scientific babble required to justify a Dimensional Crossover, and can be explained with a HandWave that it's "strange that they've never met before". These will sometimes not be in-continuity with the regular series, although that's more of the exception than the rule. '''Example''': When Marvel and DC did their ComicBook/XMen / ComicBook/TeenTitans crossover, it was done like this. Another example would be ''Series/{{Cheers}}'' characters appearing on ''Series/{{Wings}}'', and later on ''Series/{{Frasier}}''.

What kind of crossover is it? Is it a CrisisCrossover, where the characters are brought together to deal with some massive threat, often on a universal (or multiversal) scale? Is it a MassiveMultiplayerCrossover weaving in countless characters from a wide range of properties? A BatFamilyCrossover limited to a handful of tightly interconnected works?

If you're writing a MassiveMultiplayerCrossover, do all the characters inhabit the same world, or come together in a new one such as in ''VideoGame/SuperRobotWars''? Both can provide opportunities for WorldBuilding (see below in the Set Designer/Location Scout section).

How will the crossover affect the series involved? Will this be a one-shot that leaves the Status Quo of each series mostly untouched, or will there be ongoing effects that change everyone involved (for good or ill)?

Official crossovers are tricky, because so many fictional properties are owned by a wide range of corporate media interests, often giving rise to tangled and[=/=]or complex issues of copyright in the process. On the most simple level, you rarely see certain properties come together officially because different corporations own them; for example, you rarely see Comicbook/SpiderMan and Franchise/{{Batman}} hanging out because one is owned by Marvel and the other Creator/DCComics, and the DC writers don't have the right to use SpiderMan in their stories (and vice versa of course). Of course, on certain occasions the two have and can be brought together, but it's usually very rare and depends on the interest and goodwill of all copyright owners. And that's when the issue of copyright is clear-cut; certain characters exist within a legal quagmire of copyright issues.

Assuming copyright permission -- or alternatively, you're writing an unofficial fanfiction without seeking it out -- certain inherent issues within the story become apparent. When you're bringing together characters from different properties, you have to consider that these characters often have different fanbases, frequently comprising people who are fans of one but not the other. This means you have to be careful when introducing and using both properties, because chances are good that you will be aiming at an audience of which a good part will have no idea who the other character / property is. Of course, this can depend on the genre that's being brought together -- if you're bringing together two mainstream superhero properties, then it's perhaps fair to say that most of your readers will have heard of and will be able to identify both Batman and Spider-Man (in general terms at least, even if they're not familiar with the complete backstory or full details of the other character) -- but if you're bringing together two different mediums or genres, you have to be wary of treating the characters as if everyone can identify them. This is especially the case if one property is more obscure than the other. You will need to ensure that you identify and characterise them clearly so that newcomers will at least be able to gain a sense of who they are.

Certain properties may also lend themselves more to being crossed over than others, which can affect the story being crafted. To take one example, ''Series/DoctorWho'' is a property that lends itself particularly well to being crossed over with others -- the TARDIS can literally land anywhere in time and space and do anything, enabling the writer to engage with a wide-range of genres, mediums, moods, and properties; all a writer really has to do to make a crossover is plonk the TARDIS down in a particular location and have the Doctor wander around until he meets the other characters. Other properties, however, may not have the luxury of this kind of freedom for writers, being tied to a particular genre, setting, theme, etc; it would take a lot more work to effect a successful crossover wherein characters from ''Series/TheWire'' found a starship and travelled the galaxy until they met the [[Franchise/StarTrek USS Enterprise]] because the shows are quite different in genre, tone, setting, etc. Successful crossovers are aware of these limitations and manage to effectively overcome them.

Watch out for WolverinePublicity; promoting a 'crossover' between two properties which doesn't really happen, or at least doesn't happen to the degree that you're promising, is a good way of ticking off the fans of both. Also, take care to avoid a StoryBreakerTeamUp, which happens when one of the involved characters far outclasses the partner(s) or disrupts the overall tone (when it's not a parody, that is).

Depending on how often you're engaging in the crossover, ContinuityLockout can be a problem. As mentioned above, not everyone in the audience is going to be familiar with or even know both properties that are being crossed over, and not everyone in your audience for a particular series is going to enjoy or read crossovers. This means that not only do you have to be careful in using continuity in the crossover itself (so that the audience can reasonably follow what is going on without needing to be intimately familiar with everything in both properties), but if the crossover is part of a larger series you also need to be careful when referring back to it. For example, a plot element originally introduced on the show ''Series/{{Angel}}'' was transferred to the characters of ''Series/BuffyTheVampireSlayer'' via a crossover, where it ended up being vitally important in the final episode of the latter series. All very well, since both shows inhabited a SharedUniverse -- except that even with this being the case, not everyone who watched the latter also watched the former, meaning that as far as those who ''didn't'' watch both were concerned the crossover element ended up being a DeusExMachina. Of course, you can expect a certain percentage of the audience to watch both shows, but you cannot reasonably expect ''everyone'' to do so, and by using the crossover in this way you risk pissing off the people who don't -- which can be a problem if the audience for both shows has less overlap than you initially think.

Another potential pitfall is favoritism of a particular side of a crossover. This is an especially common problem in fanfiction, as authors who favor one side - or outright dislike another side - will bring the two together expressly so that the side they favor beats up on the other. This can range from a simple case of a CurbstompBattle between the two where the favored side beats down the disfavored one, or a one-sided conflict that is more moral or social in nature where one side demonstrates clear superiority over another. This can often result in a story that is {{Anvilicious}} or read like a [[WriterOnBoard writer is on board]]. This is much less common in official crossovers, as the crossover has to favor both sides equally well so as not to alienate fans of one side, but fanfiction does not have this issue and often panders to one side or the other at the expense of a good story.

!'''Potential Subversions'''
Despite bringing together two universes, this doesn't mean you have to bring the characters together so that they actually meet; you could write a story in which the characters of two properties are engaging in the same adventure from two different angles, so they don't meet up until the end -- or at all.

LetsYouAndHimFight can be a bit hackneyed, since it's a slightly cliched way of creating tension between the characters; perhaps in your crossover the characters could have a moment's friction, but actually decide to work together from the start? They could even be old friends (albeit unseen ones to other characters).

!'''Writers' Lounge'''
!!'''Suggested Themes and Aesops'''
* ContrivedCoincidence - how most characters meet, though there might be logical reasons too.
* NegativeSpaceWedgie - used to bring the characters together when the crossing series take place in distant worlds, times or universes.
* VillainTeamUp- although a single villain from one of the series can be used if he's dangerous enough for both heroes. An OriginalGeneration villain can also be used instead.
* DeconstructionCrossover - characters can pick apart each other's franchises, criticizing other characters, [[LampshadeHanging calling out weird or unpleasant elements of the others' world]], and clashing methods and styles. When done with actually legitimate criticisms, this can be quite insightful and clever.
* LetsYouAndHimFight- done mostly because fans just want to bet on who can beat whom. Usually ends on a draw in order not to piss off either fandom.
* StatusQuoIsGod- by the end everything will be as it started, since the crossover is not allowed to override the actual series. This doesn't apply in fanfiction unless the writer wants it to, though.
* FishOutOfWater - If the two universes are very different. It is good time to let a new perspective give alternative setting interaction.
* PlotTailoredToTheParty - Similarly, expect both heroes to get the chance to show off their abilities.

!!'''Potential Motifs'''
* CharacterDevelopment - A crossover provides a lot of scope for characters to experience things that they would not normally experience in their own universe. How do characters from different continuities and backgrounds influence each other? Likewise, do villains from different backgrounds complement or simply attempt to dominate each other?
* CelebrityParadox - of a sort; a lot of crossovers hinge on the fact that several characters (who are, of course, portrayed by the same performer) look rather alike, which enables them to find links between them; for example, both [[Series/{{Castle}} Richard Castle]] and [[{{Series/Firefly}} Malcolm Reynolds]] are played by the same actor (Creator/NathanFillion) and, as the former lives in an earlier time period to another, it's easy to suggest some kind of distant familial relationship or ancestry exists between the two that could be used to fuel the story (for example, Reynolds going back in time and encountering his ancestor).

!!'''Suggested Plots'''
The plot of a crossover usually takes one of these forms:
* '''The Ultimate Showdown of Ultimate Destiny''': Basically, the properties have been brought together for one hell of an almighty battle. May involve LetsYouAndHimFight (as the plot-namer in this case does) or a VillainTeamUp (or even Villain Battle[=/=]NegativeSpaceWedgie that threatens the respective worlds[=/=]realities of each property. Official crossovers tend to use this, since the rarity of the characters meeting tends to call for some kind of epic circumstances to surround it; it would be a bit underwhelming to have the Avengers and the Justice League finally meet up only for the entire plot to revolve around hunting down some random mugger, after all. The CrisisCrossover may also be involved here as well.
* '''Overlapping Adventures''': A smaller scale version of the Ultimate Showdown where the characters from one franchise happen to be working on the same (or a similar) problem to the characters from another franchise, only for the two to eventually collide; for example, Detectives Briscoe and Logan from ''Series/LawAndOrder'' might be investigating a murder only for Agents Mulder and Scully from ''Series/TheXFiles'' to be investigating the same crime, or the crew of the [[Franchise/StarTrek USS Enterprise]] might be investigating a suspicious phenomenon on a distant planet while at the same time [[Series/DoctorWho the TARDIS]] has arrived there. Official crossovers on a smaller scale to the Ultimate Showdown might use this.
* '''Two Characters Walk into a Bar''': A crossover which centres around two (or more) characters who just happen, for whatever reason, to cross paths over the course of their day-to-day lives. Often done on a smaller scale than the Overlapping Adventures, and usually the hallmark of shorter works of fanfiction. The typical example tends to involve a character from one franchise going into a bar at the end of their day and striking up a conversation with whoever they end up sitting next to, who turns out to be a character from another franchise. The focus is usually on CharacterDevelopment and Character Exploration, usually centred around VillainsOutShopping or HeroesGoneFishing; how do these two characters spark off each other? Naturally, it doesn't ''have'' to be a bar, but it's generally just some similar small, every day activity which throws these two together rather than a crisis as above (although the crisis might form the background to the story).
* '''Ten Times...''': A specific form of ensemble story common to fanfiction in which a certain number of properties (usually five or ten) are brought together around a certain theme or connection, with the story told in a number of different segments each focussing on a different property. Often, there is one "overarching" property which is then linked to the others, which otherwise might not meet up; alternatively, each segment might focus on a different property entirely, with the only connection being this overall theme. Named because the usual title[=/=]summary for this story tends to be along the lines of "Ten Times Character a Met[=/=]Did This."
* '''Hi Joe, How've You Been?''': Essentially, a crossover wherein two characters -- often one from each property -- who have a previous acquaintance or relationship of some sort meet again, often to then engage in a variation of the "Two Characters Walk Into A Bar" or "Overlapping Adventures" examples. Can often be seen in SharedUniverse properties, both because a shared audience (or at least an audience reasonably familiar with both properties) can be expected and because it makes the circumstances of the crossover easy to expect -- your readers are going to understand that since Batman and Superman exist in the same universe, have roughly the same vocation and share many of the same principles, that even though their stories are mainly told in separate comic books it's reasonable that they would know each other and cross paths on occasion.

!!'''Set Designer'''[=/=]'''Location Scout'''
One common link that makes crossovers easier to manage is if all involved franchises take place within the same basic location; it is, after all, easy to accept that characters from one franchise set in London might conceivably encounter characters from another which is also set in that city. If this is not an option, then a choice might have to be made about which franchise is going to be placed into the 'world' of the other. Depending on what the franchises are, this might be an easy choice -- a property in which the main characters are WalkingTheEarth means that they can conceivably walk into a property which is bound to one location and then walk away again when the story is done.

If it's a MassiveMultiplayerCrossover, there's some fun WorldBuilding to be had. ''VideoGame/SuperRobotWars'' created one whole new world and placed all the characters into it. Alternatively, you can say that all the stories took place in the same world from the start and create a world where they all have had impact, such as in ''ComicBook/TheLeagueOfExtraordinaryGentlemen''. The latter method can be more limiting, but can also be easier to build off of. Just look at all the properties and explore what implications they have for the wider world, and put the pieces together.

!!'''Props Department'''
The PlotTailoredToTheParty usually comes into play here; since the point of a crossover is to see the characters interact, this usually means that each character will get a chance to show off their usual skills or {{Iconic Item}}s. Also, Technology[=/=]{{Magitek}} from series A handled by characters from Series B is usually good for some laughs.

On the other hand, props should usually be limited to those held in common with the focus series and/or the neutral setting. i.e. [[TheLordOfTheRings Frodo Baggins]] and Music/MileyCyrus should not be handling [[Franchise/{{Digimon}} Digivices]] unless that series is ''also'' involved [[FusionFic in some way]].

!!'''Costume Designer'''
Usually, most crossovers don't have to think about costumes, as the standards of the Location take precedence, followed by the characters' ordinary LimitedWardrobe. However, a humorous (or {{FanService}}y) moment can be had when certain characters have to dress in a certain way, usually either to fit in with the aforementioned setting, or as a visual ActorAllusion. [[note]]Other reasons for dressing up (see {{CostumeTropes}}) are handled just as well in other genres. [[/note]] If the crossover author ''does'' have reason to expand wardrobes, one must take '''great''' care to avoid CostumePorn.

!!'''Casting Director'''
You may also have to pick and choose which characters from each franchise are going to appear; if you try and include all of them, it's going to get crowded. You might wish to consider what links can be drawn between characters -- the CelebrityParadox example noted above might not be practical, but you can consider careers, backstories, etc in deciding who is going to meet whom.

On that note, the caution against the StoryBreakerTeamUp stands. However, this does not mean that, e.g. [[Manga/DragonBall Mr. Satan]] can't fight [[Series/StarTrekTheNextGeneration Worf]] in hand-to-hand; it just means that, e.g. [[SupermanStaysOutOfGotham Goku stays off the Enterprise]]. These such limitations can also allow for ADayInTheLimelight for characters that normally have less focus. But don't give such characters [[PossessionSue too much focus/power]]...

!!'''Stunt Department'''
!'''Extra Credit'''
* The novel ''Planet X'' is an officially published (though almost certainty out of [[{{Canon}} continuity]]) crossover between the ComicBook/XMen and ''Series/StarTrekTheNextGeneration''. (And, yes, it mentions [[Creator/PatrickStewart the uncanny resemblance between Xavier and Picard]]... despite being published ''before'' the movie was cast!).
** The novel [[AlternateContinuity contains references]] to comics that were published in which various groups of X-Men met characters from the Franchise/StarTrek universe, including the cast of the original show.
* The [[PoliceProcedural police procedurals]] ''Series/HomicideLifeOnTheStreet'' and ''Series/LawAndOrder'' crossed over so frequently that after Homocide ended, Det. JustForFun/JohnMunch actually became a character on ''[[Series/LawAndOrderSpecialVictimsUnit Law & Order: SVU]].''
* [[FightingGame Fighting games]] in which characters from different franchises - or even [[VideoGame/MarvelVsCapcom different companies]] - fight each other are commonplace. But ''VideoGame/MortalKombatVsDCUniverse'' actually took the rare step of coming up with a story mode and with a plot that explains ''how'' and ''why'' the characters are fighting each other, and even goes the extra mile of explaining why characters like {{Superman}}, SelfDemonstrating/TheJoker, Liu Kang, and Raiden are having fist fights [[PowerCreepPowerSeep on equal footing]].
!!'''The Greats'''
* ''Comicbook/TheLeagueOfExtraordinaryGentlemen'', both in comic and {{film}} format, is a rare example of an officially published MassiveMultiplayerCrossover (it helps that most of the central characters are from works that are at least OlderThanRadio, if not older, and thus exist in the [[PublicDomainCharacter public domain]]; works which aren't tend to be alluded to in dialogue), wherein practically every detail is taken from another work in an attempt to put together a cohesive fictional universe for all (or at least as close to all as possible) of human storytelling. The comic is by Creator/AlanMoore and Kevin O'Neal and is well worth a look.
* ''Franchise/KingdomHearts'' took two works that sounded as if they definitely ''should not'' be crossed with each other (Eastern animation meets Western animation?! ItWillNeverCatchOn!) and managed to make the final result ''great''.
* ''VideoGame/SuperRobotWars''. Each game takes numerous HumongousMecha anime, [[WorldBuilding creates a whole new world where they can coexist]], ties the original plots together (changing some plot devices if needed), and takes a look at [[AllThereInTheManual unused materials (including model kits and novel adaptions)]] to see if there is something that can be used. The result is usually good.
* ''ComicBook/JLAAvengers'' gets right everything that ''Marvel vs DC'' (see below) gets wrong. Both franchises are respected right down to established differences in each universe's physics and geography being acknowledged. The difference in power levels between the universes is incoporated into the story instead of being handwaved aside. The story even dares to admit that certain characters would easily defeat others while allowing some of them to be smart enough to ''not'' fight for no reason.
!!'''The Epic Fails'''
* Whilst ''Comicbook/TheLeagueOfExtraordinaryGentlemen'' in comic form is widely agreed to belong in the 'Greats' category, most would put the movie based on same squarely in the 'Epic Fails' column. Not only does it water down and dilute the original, it's also in many ways quite a poorly-told story.
* ''[[Franchise/MarvelUniverse Marvel]] vs [[Franchise/TheDCU DC]]'' has its moments, but is, in many ways, a textbook example of how ''not'' to do a crossover. Missteps included letting fan vote (i.e., popularity) determine who won several of the fights (infamously leading to ComicBook/{{Wolverine}} beating Lobo); seriously misjudging characters' respective [[PowerCreepPowerSeep power levels]] (Franchise/{{Superman}} vs [[ComicBook/IncredibleHulk The Hulk]]) and/or popularity (Franchise/SpiderMan vs ... the clone ComicBook/{{Superboy}}?); having any fight they couldn't figure out how to write happen offscreen (Wolverine/Lobo), end inconclusively (Darkseid/Thanos), or be decided by outside factors (Franchise/{{Batman}}[=/=]ComicBook/CaptainAmerica, ends due to a random sewer tidal wave); and downright bizarre disrespect of the characters (suggesting that Franchise/WonderWoman could lift the hammer of [[ComicBook/TheMightyThor Thor]]? Cool. Suggesting that she'd ''need'' it to put up a good fight against ComicBook/{{Storm}}... and would lose anyway? Insane).
* ''WesternAnimation/CartoonAllStarsToTheRescue''. Despite its promise, it was quite {{anvilicious}} and somewhat hypocritical - the special warned against using drugs, though it seems to have been made ''on'' drugs.
* ''WebOriginal/PoohsAdventures''. Take ''Cartoon All-Stars To The Rescue'', and make it even more insane and surreal. That's not even scratching the surface of this web-video phenomenon.