Over lunch just now, I skimmed over a story that made me think about wish-fulfillment and writing. I guess I'm using "wish-fulfillment" here as a preferred term to "kink-fulfillment" or "self-indulgence" to describe having a specific story goal you want to achieve that goes against the grain of canon in some way. I don't want to identify or talk about this particular story, but I'll spin off an example using a twist of my own AU fantasy material: say you want to have Xander become a woman and sleep with Spike. So you sit down and start writing, and the first thing you have to do is figure out how you want to get Xander womanized--your plot device--and how much time you want to spend on setting things up. There are arguments to be made for both a short and a long set-up. One the one hand, the more time you spend in development, the more likely you are to achieve plausibility, if only by burying your device in a thicket of distracting prose. On the other hand, there's a lot to be said for stupid plot tricks--cursed music box! vengeance demon! angry witch!--that slam into a character like a baseball out of left field, and let you spend your time on the real meat of the story, which is how the character reacts to the slings and arrows of girlish fortune.
So you scribble and presto, change-o, Xander's got a brand new bag. Except, a little problem here, you're an inexperienced author and you've already started writing yourself into a corner based on the device you've chosen, because you decided that Xander being a girl wasn't sufficient to intrigue Spike--forgetting that men have dicks, and therefore *anything* is sufficient, *sheep* are sufficient--or perhaps thinking that that your premise wasn't enough to make *Xander* glom onto Spike--forgetting that he's a vampire estrogen magnet of doom. So to up the angst, you have Xander enter a girl's body at the moment of his male body's death. Ouch! But not only that, you fall prey to the common pitfall of novice writers: the forced misunderstanding. This guarantees that when Girl Xander goes to tell his friends what has happened, they will tearfully reject his attempts to explain his new booty. Melodramatic grief makes the entire Scooby gang asininely deaf to the obvious, and if that doesn't work, you create some other artificial barrier--like, hmm, hey, the girl's body he's been dumped into is...Amy's! The evil witch! No one trusts *her*, right? Yah'kay.
So, forlorn and alone, Xander must turn in his time of need to Spike, who for whatever reason is strangely attracted to this girl who he finds stumbling along the road at night, shivering, in torn and dirty clothes, while a carful of boys slows down menacingly, the better to attack this poor little bird with a broken wing, attack her with their catcalls and manhoods. Chivalrously, Spike not only rescues her, he takes her back to his crypt, and tidies her up, and dresses her in spare, stolen Buffy clothes, and feeds her cookies he happened to have lying around, and then lets her curl up and sleep on his duvet, because darn it, she's just so pitiful...and yet strangely attractive.
At this point, your story is steaming merrily along, searching for the West Indies and destined instead for America, where it will kill Indians and do other bad things. Only a
In short, it's probably more likely that the gang will figure out that it's really Xander than it is that he'll--she'll! she'll!--be forced to wander bereft and weeping in ugly shoes along the streets of Sunnydale, cut off from any help or human contact whatsoever and thus headed in splendidly miserable isolation toward the manly, inevitable arms of Spike.
And then, yes, you do have to break Anya and Xander up, and have it make sense. And you have to distract Spike from Buffy, and you need Xander to look at Spike differently. It's a hard uphill slog, because these are the characters and you can't just use them as your puppets--they want to follow their own hearts and minds, and they have years of formative experience and habit and inertia guiding them. And this is what writing is about.
Except of course, you can sometimes just get them drunk and have them fuck.
Anyway, in summary: wish-fulfillment is okay--we all write what we want to read--but the more obvious traces of it that you leave in a story, like lumber scraps around a construction site, the more obvious it is that you may need to visit the Official Buffy and Angel Fanfiction University and let Spike slap you around for a bit.
Which actually sounds like fun, now that I think about it.