We're halfway through the season (unless we're making season one) and we've covered many of the real classic episode types, with a couple of exceptions. And here's one.
A two-parter generally marks the Big Bad stepping its game up, or some other change to the status quo. Welcome To The Hellmouth introduced the status quo and changed it, What's My Line? brought in a second Vampire Slayer and Surprise and Innocence and then Becoming were all pretty big news too.
So what merits a double-size session in the middle of a season?
Things Change - This can happen in any episode, even the most seemingly innocuous filler MOTW. So given all the weird stuff that can happen in any given episode, an extra level of changiness is required. This change sticks, as well. Look at things you'd like to do different and plot hooks you haven't pulled on lately. Is the Big Bad not as big and bad as you intended? Turns out he was a fake Big Bad, the real one is way worse. Is that romance going too well? Turns out the nice sweet-natured guy is really a werewolf. Watcher too fatherly? Fire him.
Things Change For The Cast Too - Try and line up character development moments for the Cast as well. What's My Line? had Buffy questioning her role even before a much keener slayer turned up, and Surprise and Innocence was a huge change for her, and not just her, all the group dynamics moved.
High Stakes - The Big Bad makes a major move, or a bigger-than-average MOTW (technically MOTF, Monster Of The Fortnight) threatens what we know, and may succeed in smashing something up. Very possibly an important something. If you generally restrict yourself to a TV budget, now's the time to do something rather impractical. Small army of vampires? Giant monster attack? Pirate ghost ship?
A Cliffhanger - Ideally a good big hurty multiple cliffhanger. A straight "arg, we could all be killed" cliffhanger works fine, but try and throw in some other development as well. Buffy's about to be bitten by a vampire! (Back when that seemed like a real threat.) That girl isn't one of the horde of monster assassins out to kill Buffy, she's another slayer! The Judge is here and Spike and Dru have him and Buffy and Angel just did it and now he doesn't look at all well! The world's about to end and Spike's plotting and Giles has been kidnapped and Kendra's dead and Buffy's been arrested and -
One thing I'll note about cliffhangers - most players will not let them sit. In most of the cliffhanger-y writeups in The Watch House the end of the session was actually around the pre-credits of part two, because the players wouldn't stop. If you really want to try it, pack your bags before saying "to be continued" and then run for the door.
Example: Spin Around
The Siren moves directly against the Cast - just as Hope manages to secure a major gig so the last thing she wants to hear is "sorry, I have to hunt a monster" - just as Adam's ex walks in on him and Billie sharing their first kiss - just as Josh's girlfriend dumps him - just as -