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 TropesWhat 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, ChoicesWho 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 Crossover Index, but there are basically two types of crossovers.
PitfallsOfficial 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 affect 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 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 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 Lockout 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 SubversionsDespite 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? They could even be old friends (albeit unseen ones to other characters).
Suggested Themes and Aesops
Suggested PlotsThe plot of a crossover usually takes one of these forms:
Set Designer/Location ScoutOne 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 DepartmentThe 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 DesignerUsually, 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. note If the crossover author does have reason to expand wardrobes, one must take great care to avoid Costume Porn.
Casting DirectorYou 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 whom. 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...
The Epic Fails