This conversation takes a while to get going and sounds a bit confused. Really we all need to cultivate flexibility from a very early age in all the joints for a good vibrato and each of us has a unique system of limb lengths, or amount of fatty tissue at the end of the fingertip as well as flexibility so those all have to be controlled to some degree to make a sound that isn't too wobbly and not too stiff. The adjustments that Greenhouse mentions are rather personal ones. Paul seems to try and clear things up or at least make the conversation more general after about five minutes when they talk about the vibrato being "intuitive" and brings up how the bow makes the sound and how left and right work together ("an enormous other thing"). Let's not forget the bow when we talk about sound!

I believe very strongly that what really separates a player with a great sound like Greenhouse from some lesser ones is his enormous musical personality which I would call his "ear". What I mean is that he's got a great sense of musical perception in order to understand the repertoire and the instrument. People like him preserve the quality of sound as they become older and degenerative changes to the musculo-skeletal and neurological systems occur, by adapting to those changes. This is a constant process that they have developed by observing their own playing over a lifetime. Of course he's got a very strong and very convincing mental concept of the sound in the first place. He's constantly adapting his technique in ways even he is labored to explain. This also holds true for all the musical aspects besides color such as articulation, but also more general musical elements like tempo, pacing, relative dynamics, phrasing, ensemble and how all of these are prioritized. You listen to a Greenhouse or watch him teach and he always knows what he wants, what the score calls for and where it is taking the listener. So whatever was going on with the changes in his arm, joints, shoulder, wrist and hand he always knew how it was supposed to sound, so he changed whatever he had to to make it sound that way. Now, some other cellists may have turned to conducting when these things occurred, but I don't recall he did that. At any rate, getting older should be fun. YMMV.