Fanfiction is a study in what works and what doesn’t, and how something that really seems like it shouldn’t work actually does if you do it in a particular way. You get a lot of that in Ship Fanfiction, where most of the canon plot is background, and the focus of the fic is the particular Ship it’s exploring. There’s a LOT of Dramione out there that is really terrible. Clearly that ship is a bad idea. But if you just switch a thing or two around, dim some canon details, brighten others, change the levels of the picture it’s painting… and then write well, you can actually make it appealing and have it not only work, but also be quite genuine in its execution.
(A side note: Harry Potter fanficdom is by far the largest, so despite that universe not being as good as others, and in fact being quite flawed, it’s where there’s a lot of different examples of the patterns and interests of fanfic.)
Another thing goes back to fanfic being social: fanfic is often meta. Not just in the “this fanfic refers or hints or has homages to other fanfics” way. But because of the way previous fanfic has already explored many different concepts, a fanfic writer will have a general sense of fanfic tropes. There’s stuff to avoid, there’s stuff that’s already been done a thousand times so it can be safe to use if you need to have a filler, there’s stuff that’s generally said to be impossible to do, so you attempt it anyway because you want to see if you can do it. There’s community writing challenges, where you get a concept and play with it, or try to write it in.
Fanfic authors, most of the time, are aware of the context. They’re aware of the tropes. They’re aware of the trends. They’re aware of Murphy’s Law. And they can play with that. Fiction (not fanfic) writers tend to not do that. Except for a particular genre of comedy, which is somehow exempt of that limitation.
To use one of the classic examples: If, in a fanfic, someone states “I won’t die.” there is a more-than-even chance that the speaker will, or will come close to it, just because the words were said. But the writer knows that. And you, as a reader, know that the writer knows that. So now, the expectation is that the words being there causing the action is too obvious. So it won’t happen. Or there’ll be a twist in some way. Or maybe it happens, but only off-screen. Fanfic writers, more often than other writers, subvert tropes, instead of only writing them.
(And that’s an argument for non-fanfiction writers reading more fanfiction: so they then are inspired to break tropes and make chaos, to not always write the same whitewashed hero and conventional romances, to not always have Bad Romance and Good Romance, but something in between, or something that transcends it, or something that perverts it. And there’s writers doing that, sure, but they’re too few and far between, I think.)
I see it a bit like the classical composers, who (according to Dad) would compose something and then send it to their contemporaries and be like “look at I did, can you improve on that?” and the recipients would create Variations on a Theme, and tease out patterns that are interesting in their own right, and then compose something around that and send it back. Or maybe take a minor voice and make it major, take that movement that tells the story of the calm and long lamentations of the Winter Lady and make it instead about the fast wailing of a scorned lover, carve out the pattern the cellos are playing and bring it to the fore into a concerto, completely eliminate the melody and see what stands out… Essentially doing nothing but recursive fanmusic, and then what little would get out to the public would sound “so original, what a delight”… “here, have a bunch of money to keep doing what you’re doing”.
Another thing: while there is fanfic that keeps to the tone of canon, that is actually a particular style, a challenge in fact. How to write something that is yours while also following down to the beat and down to the semitone the cadence and key of the original? The rest of fanfiction doesn’t follow canon, and just… plays in the sandbox left by the original creator. It puts concrete in the sand and erects impossible spires, it attempts to break out of the walls, and it even invites itself in other sandboxes, bringing some of its sand and some of its tools over to a completely new universe. But it’s all a lot easier to get started and get writing and get exploring than building an entirely new sandbox by yourself. It’s a lot less emotionally taxing, and that translates as freedom to do a lot more.
Not that it means fanfic is always easy to write, I say as I add notes to my detailed timeline of my Star Wars AU, which helpfully indicates, in an intricate weaving of lines and colours, which parts are taken from canon, which are mine, which refer to what other fanfiction, which are from Legends, which are well-defined, which are up in the air. You can plan as little or as much as you want, there’s no worldbuilding nor character development necessary — only if you want to do it. And I like that about the medium.
[This was several responses I posted in Merveilles, then edited together and published properly here.]