Another of the archetypal episodes, where we find that magic is capricious and troublesome, often in a funny way, and particularly when applied to Cast Members.
These mostly happen to Xander.
Sometimes they hit somebody else (like Giles in A New Man) or everybody, as seen back to back in Once More With Feeling and Tabula Rasa. Some of them abruptly segue into a different plot type entirely, causing dangerous narrative whiplash, and I only recommend this for the brave amongst you.
The kind of magical effect will define the kind of comedy, and how much work is involved. Once More With Feeling would be rather difficult to do with weeks of preparation, let alone with the players getting no warning. And you'd also need to plot and scheme with players in advance to pull off something like Fred's audition scene for Angel where Wesley and Gunn start "talking Elizabethan crazy talk".
But a "play characters a bit differently" spell like "all the girls are in love with Xander" in Bewitched, Bothered And Bewildered would be less of a stretch, as would "one character is changed amusingly and possibly dangerously" like Fyarl Demon Giles. You could spring those with little or no warning.
The most planning involves making sure it gets as out of control as possible. Amy and Buffy fighting over Xander, the Cast losing their memories as they're about to be besieged by vampire mobsters...
Since these stories are about unpredictable effects be prepared for extra-very-unpredictable effects, like Willow's what-she-says-goes spell in Something Blue (which were I GMing I'd set up in advance with Willow's player so she says plenty of problematic things) nearly leading to a marriage, or Xander's love spell affecting Drusilla as well. The stranger and harder-to-explain-later, the better.
Serious stories about the abuse of power also occur, but this is mainly a light show, so play at least some of these for laughs. Farce, slapstick - look at magical comedies outside of the Buffyverse too, entire sitcoms about witches and genies run on this.
And if there's a sorcerer among the player characters, one of these might happen when you least expect it. The Watch House episode Secret Identity was entirely the players' idea, beyond the initial setup of Matthew being jealous of Natalie and Thomas...
Example: Sugar And Spice
Bass player and girl trouble magnet Josh is nearly settled with a new girlfriend, much to the annoyance of a recently jilted girlfriend who goes to the local magic shop and buys a curse so that he'll know how it feels for a woman...
(I okayed this with Josh's player, since it throws him (or rather her) into the spotlight in a potentially cringe-inducing manner. He responded: "You're cruel. I like that in a GM.")
The spell naturally hits while he's with the sure-to-be-cruelly-amused Hope rather than the will-help-sensibly Billie, and he was with Hope because he was shopping so she'll be sure to play with that, and that's because he needed a new shirt before a big date with the unaware-of-all-this new girlfriend, and while there's a monster targeting women on campus who already got too good a view of the band for their female members to be able to work as bait, and...