The best “poker face” for bluffing may not be a neutral face, but rather a face that contains emotional correlates of trustworthiness. Moreover, it suggests that rapid impressions of an opponent play an important role in competitive games, especially when people have little or no experience with an opponent.
-"Human Wagering Behavior Depends on Opponent's Faces"
The Freaknomics blog today mentions this study of human faces when making poker-like wagering decisions.
I meant "poker-like" because the study involved novices to Texas hold'em and uses two options of calling or folding a hand.
Obviously, true analysis of poker wagering starts with the pre-flop hand. Simply put, in many cases poker writers say all streets "tell a story" about the hand. Poker players often have to ask themselves if the so-called bluff is credible and thus a wagering decision may not even get to examinating of a "poker face."
Also the ratio of the bet to the pot is a determinant in whether a good poker player will call the bluff or not. There may be too much money involved to lay down a hand, even with a marginal one.
And a third tool missing here is to raise the so-called bluff. Your hand may have enough value to do so, or to even extract a laydown.
It's an interesting point though that the study seems to suggest that people who look trustworthy make better liars.
I'm currently reading Erik Larson's "The Devil in the White City" which deals in part with the 19th century serial killer H.H. Holmes and it seems apparent that his appearance and seeming trustworthiness influenced many around him.