I agree with what HeartKeys said.
Of course I do not have the definitive answer, but after spending time with recordings and working out some arrangements, the way I humbly view it is this: the harmonic framework for each song would likely be pretty well defined (chord progressions, often used substitutions, other style-istic trademarks of his). Therefore when performing, one can be in a say semi automatic mode where one can "play around Β» up to a certain point at whim within the melody constraints. It would be a little like having a toolbox where one can pull out different tricks to suit particular musical contexts at specific points.
I may be totally out to lunch on this, btw In any case this is what I strive to do when I dabble at the keyboard in general; I seem to have certain hand imprints or accompaniment sequences which I naturally gravitate towards without having to think about it too much. Of course, my problem is that I do not have all that wide a range of such reflexes and I fall flat into the trap of sounding too repetitive, grrrβ¦..