So You Want To/Write a Crossover

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 Write a Story for writing advice that transcends genre, in order to make your story the best it can possibly be. See also Write a Fanfic, 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 Crossover Index, 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.

Let's You and Him Fight 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.

Villain Team Up 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?

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

Pitfalls
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 Spider Man and Batman hanging out because one is owned by Marvel and the other DC Comics, and the DC writers don't have the right to use Spider Man 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 effect the story being crafted. To take one example, Doctor Who 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 effect 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 The Wire found a starship and travelled the galaxy until they met the 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 Wolverine Publicity; 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 Story Breaker Team Up, 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, Continuity Lock Out 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 Angel was transferred to the characters of Buffy the Vampire Slayer 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 Shared Universe -- 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 Deus Ex Machina. 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.

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.

Let's You and Him Fight 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? Or even old friends (albeit unseen ones to other characters)

Suggested Themes and Aesops

 * Contrived Coincidence - how most characters meet, though there might be logical reasons too.
 * Negative Space Wedgie - used to bring the characters together when the crossing series take place in distant worlds, times or universes.
 * Villain Team Up- although a single villain from one of the series can be used if he's dangerous enough for both heroes. An Original Generation villain can also be used instead.
 * Let's You and Him Fight- 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.
 * Status Quo Is God- by then 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, tho.
 * Fish Out of Water - If the two universes are very different. It is good time to let a new perspective give alternative setting interaction.
 * Plot Tailored to The Party - Similarly, expect both heroes to get the chance to show off their abilities.

Potential Motifs

 * Character Development - 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?
 * Celebrity Paradox - 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, perhaps Richard Castle is a direct ancestor of Malcolm Reynolds?

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 Let's You and Him Fight (as the plot-namer in this case does) or a Villain Team Up (or even Villain Battle / Negative Space Wedgie 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 Crisis Crossover 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 Law and Order might be investigating a murder only for Agents Mulder and Scully from The X Files to be investigating the same crime, or the crew of the USS Enterprise might be investigating a suspicious phenomenon on a distant planet while at the same time 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 Character Development and Character Exploration, usually centred around Villains Out Shopping or Heroes Gone Fishing; 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 Shared Universe 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 Walking the Earth 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.

Props Department
The Plot Tailored to The Party 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 Items. 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. Frodo Baggins and Miley Cyrus should not be handling Digivices unless that series is also involved 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 Limited Wardrobe. However, a humorous (or FanServicey) 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 Actor Allusion. If the crossover author does have reason to expand wardrobes, one must take great care to avoid Costume Porn.

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 Celebrity Paradox example noted above might not be practical, but you can consider careers, backstories, etc in deciding who is going to meet who.

On that note, the caution against the Story Breaker Team Up stands. However, this does not mean that, e.g. Mr. Satan can't fight Worf in hand-to-hand; it just means that, e.g. Goku stays off the Enterprise. These such limitations can also allow for A Day in The Limelight for characters that normally have less focus. But don't give such characters too much focus/power...

Extra Credit

 * The novel Planet X is an officially published (though almost certainy out of continuity) crossover between the X Men and Star Trek the Next Generation. Not as bad as it sounds. (And, yes, it mentions the uncanny resemblance between Xavier and Picard... despite being published before the movie was cast!).
 * The novel contains references to comics that were published in which various groups of X-Men met characters from the Star Trek universe, including the cast of the original show.
 * The police procedurals Homicide: Life on the Street and Law and Order crossed over so frequently that after Homocide ended, Det. John Munch actually became a character on Law & Order:SVU.
 * Fighting games in which characters from different franchises - or even different companies - fight each other are commonplace. But Mortal Kombat vs. DC Universe 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, The Joker, Liu Kang, and Raiden are having fist fights on equal footing.

The Greats

 * The League of Extraordinary Gentlemen, both in comic and Film format, is a rare example of an officially published Massive Multiplayer Crossover (it helps that most of the central characters are from works that are at least Older Than Radio, if not older, and thus exist in the 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 Alan Moore and Kevin O'Neal and is well worth a look.
 * Kingdom Hearts took two works that sounded as if they definitely should not be crossed with each other (Eastern animation meets Western animation?! It Will Never Catch On!) and managed to make the final result great.
 * Super Robot Wars. Each game takes numerous Humongous Mecha anime, 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 unused materials (including model kits and novel adaptions) to see if there is something that can be used. The result is usually good.
 * JLA/Avengers 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 The League of Extraordinary Gentlemen 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.
 * Marvel vs 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 Wolverine beating Lobo); seriously misjudging characters' respective power levels (Superman vs The Hulk) and/or popularity (Spider Man vs ... the clone 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 (Batman/CaptainAmerica, ends due to a random sewer tidal wave); and downright bizarre disrespect of the characters (suggesting that Wonder Woman could lift the hammer of Thor? Cool. Suggesting that she'd need it to put up a good fight against Storm of the X Men ... and would lose anyway? Insane).