I attended a 2 hour workshop and house concert with David Grier this past weekend. I learned a lot about David’s process of learning and developing a tune.
Let me first say he is a heck of a nice fellow. And it seemed to me that he has spent an extreme amount of time with his guitar in his life. I say this because:
His playing is spectacular
He does not read music
He does not read tab
Claims he does not know music theory (although I would bet his ears know quite a bit)
He learns everything by ear/memory
The way he discovers things on the guitar (chord movement, note movement) points to someone who questions everything he does on the instrument and uses those questions toward more discovery (huge amounts of time spent).
And if you want to hear a joke (ones not for mixed company), spend some time with him:
“Did ya hear the one about the fella who had 5 penises? His pants fit like a glove!”
This was one of the clean jokes! 
He writes a lot of music but rarely names his tunes. And the tunes he writes contain beautiful melodies and harmonies. Honestly, I was not sure I would be comfortable with a full evening of just solo guitar. After all, I have heard some of the best ever in this sort of setting (Joe Pass, Angel Romero, John Williams…etc) and always found myself feeling itchy to go at some point in the evening. This was not the case with David. His playing was always engaging, surprising, harmonically interesting and fun to listen to. And he ended the evening with the most amazing version of Freight Train I have ever heard, it was nothing short of unbelievable.
If you get a chance to see him either in a group or solo, by all means go!