Thanks Bill, for the following: Well, I don't know. It seems to me that session music--i.e. Celtic--is fairly structured by nature, and most good sessions don't just happen--they are more or less "owned" by someone, whose attitude controls the vibe.
I think there is a difference between the music being structured and the session being so. It's true that there is more structure in the music when you are playing Irish/Scottish/English tunes as they have a/b parts and the tunes follow on etc; the melody players need really to know the tune being played. The same is true to some extent for Old-Time/Bluegrass.
This is not so important when it is a Blues or Jazz jam when it's enough to only know the groove or riff being played and improvisation is the rule. There is also more room for accomodation when it is a mainly song based session.
It's open to debate what exactly is a good session. In my opinion the best ones do appear to just happen and the ones that are obviously 'owned' tend to be possibly better musically, but lose the 'something' I seem to prefer. Maybe 'guided' is a better word?
In my experience musicians do not need much encouragement to play, just get them together, (in a bar) and they will take it from there. (They might just play too).
The other point I would like to reply to is: As in other walks of life, its not usually enjoyable receiving criticism on your musical tendencies, but that's how you grow as a musician.
I think you would agree that is is better to come to a session to learn, make mistakes and get advice than to read notices that prevent you playing at all?