Our heroes have been stomping the forces of evil for a while now. They may even have special stomping the forces of evil boots. They might have a growing sense of ignorance being bliss, regular folks having it easy and the like. They might not think that much about it, though.
Time to show them just how strange their routine is.
The main way to do this is to have someone (a new PC or a potentially major NPC) come in who is or would be at least as weirded out by the discovery that vampires are real as Willow and Xander were back in the day. And the Cast have to make sure this doesn't get the new guy, them, and possibly everyone else killed.
A lot of folks handle introduction to the terrors of the Buffyverse pretty well, but some don't. We first see this with Owen in Never Kill A Boy On The First Date, And while Joyce copes fairly well, when Gingerbread comes around she's the one that falls under the spell. Xander and Giles both lose possible love interests over this. And then there's Kate in Angel...
Outside perspective can also come from the other direction - those involved in supernatural business, who can appear just about as strange to them as they do to the "straights" and act as cautionary tales for the more extreme go-for-it PCs. Kendra's a good example here, a slayer who's all about the slaying. Maybe the Cast can get them to chill a bit. Maybe they can't.
So take a look at the characters' situation, their attitudes to the fight and to those they're fighting for. If they're getting a tad blasé about the whole thing, here we go.
Example: Thicker Than Blood
Hope's sister Joy visits, down in the dumps following a disastrous breakup, just as the gang has to stop endless darkness consuming the Earth. Keeping her unaware of the supernatural would involve a lot of farce-style hiding of inconvenient monster attacks, especially when she takes an interest in one of the boys in the band...