Both bluegrass and Dixieland jazz have this similarity: There is a central theme to the music being played, but all participants are playing almost (but not quite) free-style improvization around that theme. They take turns in presenting either the direct theme or complex variations on that theme.
I say "almost but not quite free-style" because the odds are extremely great that the same group playing the same selection on two consecutive days are unlikely to play it the same way both times. And I'm not talking about little imperfections. I'm talking about different apoggiatura, grace notes, and other musical decorations.
The selection should easily be recognizable on each of the two days, but it won't be quite the same. Compare and contrast to a performance of a philharmonic symphony orchestra. In the latter case, every note must be precisely played as the conductor desires it. In Dixieland or bluegrass, they play by stylistic agreement, not precise note-by-note rehearsals.
Just thought I'd toss in a comment on something I actually understand. I have in the past played Dixieland jazz on Bourbon Street of New Orleans. Did that in my college days before I got an honest job. (Well, at least relatively honest...)
