My take on the "stone-face" question:

A big part of what I like best about my favorite kinds of folk music is a characteristically low-key, no-show-biz approach. You certainly find this quality in most acoustic blues, old and new, as well as various strains of mountain/hillbilly music. (In other words, both black and white American "roots" music.)

The same expression of fellow humanity (as opposed to that of a performer separate from an audience) seems to be a common quality of folk music from the rest of the world as well.

The highly stoic presentation that has become so common among bluegrass players is only the most extreme example of this anti-histrionic attitude. Sometimes, however, I get the feeling that such an over-the-top display of UN-feeling may itself be a kind of playacting.