how would you GM something like a "shapeshifters impersonating the PCs" situation?

[Dalin]

When this has come up over the years in face-to-face games, I've always taken advantage of a break or the end of a session to talk to the player of the possessed PC. I recall one scenario where the spirit of an ancient witch queen randomly possessed the pacifist healer in the group. The character and player were so implicitly trusted that nobody suspected anything, even when she started stepping way over the line. It lasted multiple sessions and is still talked about by the players more than 20 years later.

[finn]

Originally Posted by ericbsmith

To assuage suspicions, hand everyone a note with different directions, only one of which is for the shapeshifter. For extra fun, randomize the notes so that even you don't know who the shapeshifter is.

That sounds similar to what you do when you run a Murder Mystery game. Everyone gets instructions about their character, they all know one of them is the killer, they all have different bits of information related to the murder, but only the murderer knows that they are the killer.

If I were to run a "find the imposter" game as part of an RPG campaign, I would let everyone know that I am looking for a volunteer to play an imposter, and secretly work out any details beforehand with that player.

I would let the players know that the day's game will have a "find the imposter" theme, and make sure that the characters learn that there is an imposter among them early too.

This should prevent the players needlessly doubting each other in the future.

[Pursuivant]

The only caution I'd add is that there has to be serious sweetener for a player to play in imposter who's directly sabotaging the party, much less their PC.

Some players might not be up to the roleplaying challenge. Others might have a mental block about sabotaging party cohesion or damaging their PC's achievements. Still others might take playing an imposter farther than the GM wants them to go.

I'd lay out specific character point awards and penalties, in advance, for the imposter.

E.g.,

Get to the end of the play session without revealing yourself as an impostor, +1 CP.
Keep the party from exploring the mansion's basement, +2 CP.
Keep the party from exploring the graveyard on the mansion grounds, +1 CP.
Get a party member to pick up and handle the dagger in the drawing room, +1 CP
Killing a party member, -1 CP
Incapacitating a party member, +/-0 CP.

[Plane]

Originally Posted by ericbsmith

There's an episode of Deep Space 9 "Whispers" where that happens to Miles O'Brien. Most of the episode is shot from the perspective of the imposter, so that the aundience isn't let in on the situation until near the end. Would make a good watch for anyone thinking of running this kind of scenario.

Paul Coyle sounds like a good writer

[Alden Loveshade]

Originally Posted by Plane

For example like the "Migoi" in the Sanctuary TV series episodes "Kush" and "Icebreaker".

Basically each PC knows "I'm not a shapeshifter" but they don't know if who they're interacting with is another PC or a shapeshifter.

This type of thing seems easier if it's just shapeshifters impersonating NPCs since the GM roleplays whoever you interact with.

There is a clear appeal-to-metagaming problem when it comes to impersonating other PCs though because (especially with in-person gamine) you know whether the player or GM are speaking.

In situations like this, would it make sense for the GM to request the player to not just play their own character, but also their shapeshifter NPC doppleganger, so that other PCs can't figure out who is who?

The PC shouldn't really be able to control the NPC's actions totally though, so the GM should probably tell them roughly what to do, and be passing notes and stuff.

I would think that might be easier w/ online gaming since the timing/frequency of note-passing might give away which version is the NPC impersonator in IRL games.

I've done this almost exactly as you've described it. I would lean toward having a player who's a talented roleplaying do it, which is what I did. In one case, I told the player what the impersonator was up to. There would be appropriate rolls, and on a failure the player had the impersonator do something out of character.

Another player figured out something was wrong; if I recall correctly, that player and then others suspected the PC was who they appeared to be, but were being controlled or unnaturally influenced in some way (brainwash, mind control, etc.)