Koh

[Geiger Counter]

the creature from avatar.
What kind of save must a player make to avoid making a facial expression?

[The Dark Fiddler]

I'd imagine a Will-save, or possibly a specific application of Concentration or Autohypnosis. Perhaps use Concentration and Autohypnosis as a synergy bonus to the save?

[RiOrius]

Bluff also feels relevant: the ability to keep a straight face under pressure.

Straight-up Will save would be the most D&D thing to do. Depending on your players, though, they may appreciate getting bonuses for skill ranks, etc.

[Geiger Counter]

How should I set DCs?
I imagine Koh can make use of various skills to illicit an emotional response.
Intimidate=fear. perform (comedy)= happiness. Bluff= anger.
Also there should probably be a DC just to keep a strait face without antagonization from Koh.

[Binks]

Straight face without antagonization I don't think would require a check. It requires about as much concentration as, say, casting without anything distracting you, and that doesn't require a check.

Personally I'd handle it as a simple intimidate or bluff check, same DC as intimidating/bluffing normally (which is to say the modified level save). Anyone who fails triggers an immediate spot check by Koh, opposed by either bluff or concentration for the player to represent him turning his back during his chats (like he did in the episode I think you're using as reference). If they fail that check then they lose.

Of course I'd also be hesitant to throw something like this (it's pretty close to save or die, it's basically save or lose your face, which is not very pleasant to most people) against players personally.

[Geiger Counter]

Yeah this is definitely not a random encounter It'll probably live in a deep swamp or desert under a tree. It's very old and knowledgeable, and like's to chat with people for the opportunity to steal their face. I think I'll just let the PC use their highest save.

I'm thinking that stealing someone's face makes them blind and mute, however I would grant them blindsight and they would no longer need to eat or breath, also I would allow telepathy to be gained with a feat.