There's a line between old-timey and bluegrass that, at least for me, is pretty fuzzy. Is it that 3-fingered assertive attack on banjo that makes it bluegrass? Is it a mandolin (or other instrument) that plays a fast improvisation all over the scale? I never would have thought of Grandpa Jones as bluegrass. I haven't heard that much by him, and what I've heard him play was old-timey clawhammer. Then again, I've heard people play clawhammer banjo on bluegrass songs. Fuzzy line.

As to the immobility of some, it may just come naturally to whoever's playing, maybe they're just concentrating on their fingers, or it may be a style they picked up from emulating someone else. What gets me is how those guys can sing without hardly opening their mouths. (There was a local band "Lonesome Dave and the North Dixie Road Kings" that did a song called "Singin' Through My Nose.")