Characters: Buffy, Spike
I have a great love of mind switch stories and plots, and while I've read a few very good ones involving Spike, there is a "logistic" matter that I've never really seen addressed (or only lightly glossed over): Spike's demon would need to stay put, or the body would probably die when the mind switch happen. Human souls or minds by themselves can't animate a corpse (or we'd be a lot harder to kill).
There are a couple of ways to take a story that runs on this premise, but for this particular challenge: the kind of mind switch story I'm really after separates both the demon from the mind/personality and the "soul" from the mind/personality (after all, the Mayor had no soul, and although he /became/ a demon, most of the time he didn't /have/ a demon). So what I'm looking for is a story where Spike's mind/personality are swapped with Buffy's, but the demon and soul stay in their original bodies.
Buffy would have to deal not only with Spike's demon, but wouldn't have her own soul as a moderating influence, while Spike, aside from having to grapple with the empathy (and probable guilt), would also be faced with the question of just how much of him comes from the demon and how much is just... him (if you look at some of the Season 10 development, Angel, Spike, Drusilla, etc, all essentially have the same 'demon', which helps to show just how different two vampires with the same 'demon' can be). Also, since demons are staying with their original bodies, the demonic Slayer-powers would stay with Buffy's body.
I would like to see this taking place sometime before Spike actually gets his soul, so we're looking at a story set somewhere during seasons 2-6. I really view this as being more pre-Spuffy than actual Spuffy, although obviously if it takes place in S6, they have their relationship then to contend with (I know it's not comfortable, but if you make it during S6, please keep their relationship as unhealthy as it is in canon). Putting it very early on means that Buffy's friends (and probably Spike) would want to find a way to keep Buffy-without-a-soul-in-Spike's-vampiric-body from killing, but would also mean dealing with whatever was going on in canon at a time (is Spike still with Drusilla? Is he stuck in a wheel chair? Has Angel lost his soul and become Angelus? ... that's a heck of a can of worms, isn't it?). Putting it in S4-S6 means that Buffy-in-Spike's-body would have the chip to keep her from killing (which would be a relief when she's back in her own body).
I would like at least a bit of a plot involved, as well as an explanation for why the switch occurs (although it could be something as simple as the end of "This Year's Girl" going very differently), and a few scenes exploring the two having to deal with their new situation, and the reactions of those around them.