When I started studying with Bob I was deep into Lee Konitz. I was aware that Bob came out of the same generation and pool of musicians, meaning both were post Basie/Lester Young players associated with the "cool school."
I did my best to sound soft, airy, and hip in the first lesson. He wasn't having it.
He told me to play a Bb major scale - the first scale I ever learned - as loud as I could. Then he asked me to do it again, then again, then again. Nope, never got it right that first lesson.
I couldn't hear what he was listening for yet. I later figured out that he wanted me to play each note with as much volume as the last, playing even and not backing off at all.
He wanted me to play with conviction.
He swore Lee Konitz was a loud player. I didn't believe him but whether or not it was true (I now know that it is!) wasn't the point.
The point was that he wanted me to believe in my ideas. When you play loud, you are forced to except what comes out.
He said something similar about sight-reading and intonation - you gotta look ahead to that note that you are dreading and love it!
It's hard to name a great musician who doesn't sound like they have belief and sincerity in what they are doing in the moment.
I think of Bob's lines. They have a way of sitting there, in their own lyricism, swinging like hell.
It takes guts to play that beautifully.