Here comes a weighty generalization: A pianist who has not been immersed in Schubert songs probably has no clue as to how he wrote for the piano.
A singer does not sing what most of us think of as notes. Anyone who has worked consistently--on a day-by-day basis--with a singer knows that there is more to it than that, as the overtones of the voice are subject to many variables, from atmospheric to sinus conditions. Playing the piano with a singer requires constant attention to balances within the sound as the shifting vocal overtones, sometimes high, sometimes low, invite new and unusual voicings of the piano part. It is inherently dramatic, never a matter of active song plus passive accompaniment.
Extending the sense of overtone-based sound to Schubert's solo piano works may seem like a leap into the void, but it is actually quite the opposite. For many of the most puzzling passages in Schubert are densely voiced so as to conceal melodic lines hidden in the inner voices. Unearthing them is a matter of listening to the whole sound, rather than trying to parse the passage theoretically.
No amount of theory can account for the constant changing of the human voice. Two experiences come to mind.
In a recital of a soprano doing Schubert at Alice Tully Hall some years ago the accompaniments, all very familiar to me, sounded entirely new. Then I realized that the singer had a particularly high-end overtone ring which the pianist was picking up and enriching. It was thrilling.
The other experience played out over a period of time: A usually sensitive professional student in a performance class I was coaching began to sound desultory. Upon questioning about his practice routine it turned out he had taken to working on an electronic keyboard so that he could play late into the night. I was amazed at how audibly this had dulled his piano playing. When, a few years later, he auditioned to take the course playing duos with a very fine clarinetist, his playing was totally transformed, this time in the right direction. Upon questioning, it turned out that in the meantime he had been working as a vocal accompanist. That would do it. His fine ear and quick response were completely restored.
