Possession, body swaps, misfiring magic, curses, amnesia, evil twins, an existing alter ego like the demon inside an ensouled vampire taking over, de-evolution beer, everybody turning out to be an actor in a TV series...
The One Where Everybody Acts Out Of Character is a staple of the genre, often done every season or two to give the actors and fans a bit of fun.
They can be completely different beings from the regular PCs, or warped reflections or exaggerated aspects of their true personalities. Magically-Paranoid Buffy is different from Vampire Buffy is different from Possessed Buffy.
Whatever the cause, The One Where Everybody Acts Out Of Character lets you play a different game in the same setting with the same players and sort of the same characters, which can be a lot of fun if the players are so inclined.
Warning: Do not do The One Where Everybody Acts Out Of Character in the first few sessions of a game. We need to get to know these characters so we know they're acting out of character. One Cast Member acting out of character after a couple episodes is fine, and easily hidden. But doing The One Where Everybody Acts Out Of Character as your second regular episode will just confuse people. I'm looking at you, Star Trek: The Next Generation.
Despite the name, The One Where Everybody Acts Out Of Character need not involve everybody acting out of character. Having one or two sane PCs in a world gone mad is often essential to stopping the effect.
The new personalities can solve the problem as well - like the ghosts in I Only Have Eyes For You who just need a chance to talk, and so they possess superhuman bodies that won't die while they argue.
To be properly playable, the alterna-selves need their own agendas as well as fun shticks to mess around with.
Besides how and why Everybody Acts Out Of Character, the main question to consider here is whether to tell the players, or some of them, in advance.
If they have to be sly about things, prep them - so if one of the Cast has been replaced by a doppelganger with a dangerous secret agenda, tell the player in advance and work out some plans for how this can revealed at a suitably dramatic moment.
If you're not sure they'll go for it, ask ahead. I find asking "do you want to be mind controlled and attack the party?" gets better results than saying "you're mind controlled and attack the party" does.
If the new personalities reflect or reveal secrets about their real selves, discuss those in advance.
If you want showboating scenery-chewing performances, generally best to give them time to rehearse.
If you want to throw everybody in at the deep end, this can work, but generally works better with the same characters in a different setting.
Example: Creative Differences
The band signs to record an album... and their style isn't quite what the producer wants... so the producer starts to change them.
(Yes, it is the plot of Josie And The Pussycats. If you haven't seen it, consider it research.)