Harmonica/Guitar duets

Most of it is worked out without sheet music. Notice I didn't say improvised. That happens, too, of course. But music  can be worked out - arranged if you like - by playing together, listening, and finding stuff that works, maybe discussing it.

A big part of working things out through actually doing it is having a common understanding of a known style. In blues, of course, there are plenty of models - Muddy and Walter, Sonny and Brownie, Big WAlter with Johnny Shines or Jimmy Rogers, and so on. A lot of what you hear is elaborations on those foundations.

Certain guitar players know what works well with harmoncia players, largely but not exclusively through studying those models. Guys like Kid Anderson, Rusty Zinn, Charlie Baty, and even Shawn Starski are guys who both know the tradition of accompanying harmonica players and have also worked it out through playing with people like Mark Hummel, Rick Estrin, and, on the more modern, tock and jazz oriented end of things, with Jason Ricci.

One problem with sheet music is that it simply can't capture and represent the nuances or the endless variations in style that exist in the music. It's oral and aural music that's learned through listening and knowing. Takes awhile to acquire that experience, but when you get it, you really have it.