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

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.

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

I had a group of player-characters who came across a ship where all the crew had been murdered. As they explored, I rolled randomly for one of them to become possessed by one of the lingering, vengeful spirits.

The random choice was a first-time roleplayer. I passed her a note that basically just told her, "you're possessed." She totally ran with it and did an awesome job. It was one of the more memorable bits in the entire campaign.

I would do something similar here: pass the player a note that says, "your character is out of action and you are now the shapeshifter that has taken his/her place" and trust the player to act accordingly. If need be, share a bit about the shapeshifters' goals or motivations, especially if they weren't clear to the players before.

If your players are the suspicious types, share a few trivial perceptions ("you see that the door was left unlatched") with each of the players by note over the course of a few scenes to cover up which one is actually the shapeshifter.

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

The only time I have tried to do something like this I ended up taking advantage of a break in the game to pass the player a very long note (roughly detailing what the imposter knew and what their goal was) and then trusted the player to do her part.

The effects were quite good, despite being detected she actually achieved her goals causing a whole new adventure. But I feel that I was lucky that I had a player who wouldn't use metagame knowledge and was sufficiently devious to stuff the other players over on the spur of the moment.

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

A lot will depend on what the shapeshifter is doing - if they're only doing nefarious things "off screen," you could actually leave the player who is controlling the replaced character completely in the dark, particularly if the shapeshifter is a perfect impersonator. If the shapeshifter needs to do nefarious stuff "on screen," concealing it as though it were normal behavior, that's generally going to require the player to be in the know.

Of course, a lot also depends on your players. For some players, they can sufficiently divorce player-knowledge and character-knowledge such that you could literally game out Alex Mercer murdering, absorbing, and shapeshifting into the unfortunate PC in front of all the players, and when he shows back up amongst the party, they'll have their characters behave as though it's still their friend. Others? Not so much.

If I wanted to do a "shapeshifter replaces one of the PC's" plot (and honestly, I don't think I'd want to), I'd probably let the players know ahead of time that that's a thing that may happen, then probably at the end of the first session (of that story arc) I'd have the PC's split up. After that, I'd arrange one-on-one sessions with each player to game out what they do when they're separated - and for at least one of them, part of that would be an encounter that could result in the character's death or capture, in a manner that would allow the shapeshifter to take their place. If any fail, those would be the ones I could have play as the shapeshifter in the next session (possibly the next several sessions, depending on how long the plot goes). If they all succeed, then if the shapeshifter didn't get captured/killed during one of the solo sessions, I'd have it take the place of an NPC (probably with more hints than they'd get with it taking the place of a PC, as a reward for nobody getting killed/captured).

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

One thought I had once upon a time (to be used only with a group I knew could handle it appropriately) is one character is an imposter, and all the characters except that one know it. He's a clone/robot/etc. that has the character's memories, but it's important for some reason that he not realize he is not the original.

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

Another idea is instead of the shapeshifter replacing, it just shows up and you have those "which one do I shoot" moments where to avoid metagame, the PC has to game both their PC and the shapeshifter, while at the same time being guided by the GM.

Or I guess the GM could play both but ideally you want hte player to control their own PC.

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

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.

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

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 audience 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.

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

That's brilliant. I'mma write a blog post about this. What a insanely great idea.

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

Thinking about this a little more, keeping up the secrecy and suspense will require passing secret notes back and forth between the player and GM. The easiest way I can think of to do this is for, at various points in the game, hand everyone a note card, telling everybody to write a note to the GM. On the back of the card, simply write numbers 1 to X; the players should know their number but the GM should make a conscious effort not to. Simply look at the front of the notes, respond as needed, then hand them back to the players face down. The players pick their note out of the pile, and hopefully the GM doesn't connect the player to the note. Even if you do, at least you tried, but keeping the secrecy for a while could be fun. There may be better ways to do this, but honestly this could make for a really fun game with the right group.

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

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.

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

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.

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

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.

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

Paul Coyle sounds like a good writer

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

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.)