How do you successfully put your PCs into a The Thing like scenario

>>55426834
Kind of hard to do in a long-running campaign. Much easier in a one-shot with notecards and such.

>>55426834
Pass notes to everyone at the table with stuff like "Look at me, nod, and fold this card" written on them in addition to "You are now The Thing"

>>55426984
This.

Just passing notes between people will make the group distrustful to each other, even if this is a long-runner.
Also, throw number of NPCs almost double as the number of PCs into the scenario and make ONLY ONE of them The Thing.

Even better
>MAKE IT OPEN
Declare: there is a Thing among you.
Then you put in a bag a folded paper for every player plus a folded blank one. Then you fish one randomly. So at first nobody, not even the GM, knows who the Thing is. If the fished card is revealed to be the blank, the GM can choose any NPC he likes to become the Thing.
This way every player has to act like if he was the character AND the Thing maskeraded as the character. So he has to play to minimize the risk to be attacked AND to be discovered.

This is kinda out there, but how about -

Don't tell the infected players. Play the creature as an npc working offscreen, in any infected player's offscreen time.

>>55426834
Don't tell them it's a The Thing-like scenario. Heavy note passing, isolated setting, etc. No one is a monster in the beginning, but as characters mysteriously disappear and return, you'll quickly build on the paranoia until the reveal.