I realize that on a breakneck song, with complex precision breaks that bluegrass musicians might have to put all of their attention on playing, and show no emotion. Kinda. Tell Jimmie Hendrix that. But, not every bluegrass song is played at a breakneck pace. The great stone faces show no emotion, even on the slow songs when they aren't as completely preoccupied with fast breaks. I think that it's more a matter of "attitude" with some musicians.

I've had friends in old-time (and even punk bands) and I've kidded them that the band drew straws and whoever got the shortest one had to sing. They all laughed and said that was pretty much true. One friend, who never sang when he was playing in duos with a singer, never sang. When he joined an old-time band, he became their lead singer. No one else wanted to sing. Of course, there are bluegrass bands with great singers, but I think that most musicians are excited about the instrumental energy.

The great stone faces I see in old-time and black gospel music aren't stone faced because they are doing incredible instrumental breaks. They just think it looks cool... meaning detached, and above it all.
I'm surprised they don't yawn more often.

It ain't just bluegrass, friends.

Jerry