I think it's whatever works for an individual. For someone like Joe Pass, he's probably thinking of a substitute for a substitute, whereas someone like Reinhardt just simply played without giving anything much thought in the theory department at all. ( He was probably thinking about fishing ). I have a great story about Reinhardt told to me by a leading jazz guitarist of the day who was working in a club that Reinhardt happened to be going for a drink in that night. But's that's another story another day. Many years ago though I was in a store in London with my freind who is a very good bass player and he was trying out a Fender Jazz bass. Off he goes on a lovely blusey jazzy walking bass line and I grabbed a guitar to fill things up a bit. All of a sudden the door bursts open and there is a big fat black guy in a white suit. " Yeah yeah I dig" He says beaming all over his face and sits at at a piano and joins in.. He didn't ask the key or anything and suddenly goes into a bass left hand all in octaves. We had a great jam and after about five minutes he gets up as quickly as he had sat down and shouts out.."Thanks guys, gotta go, catch ya later" and dissapears out of the store. "Jesus Christ I said to the store manager, who the fuck was that, he's an insane player?" The guy laughed and said "Don't you know who that is Phil?" " I have seen him somewhere I said, but I can't put a name to the face." "You idiot" my mate said.. " That was Oscar Peterson". Duhhhhhh!... What a great down to earth guy. No big head..No holier than thou attitude. He simply heard a groove he liked as he was passing and joined in. I have never forgotten that day because I hadn't heard piano playing like it before..Or since for that matter. I wonder what was going on in his head. It certainly wasn't..Shall I play this mode or that scale. He simply played what he felt...Awesome wasn't the word..