Thanks rkram.
The single most challenging element to bring to any theremin performance is FIRE and a sense of passion, emotion and drama. That is because the tools the instrument gives you consist solely of pitch and volume control. Unlike members of the family of viols, the theremin is incapable of double stopping, timbre control, articulation, and all sorts of other possibilities that we take for granted on traditional instruments.
What the theremin has that no other instrument comes close to, is its uncannily human soul. Without this, a theremin performance is liable to fall short of what it might have been. I think that is why Clara Rockmore chose to use only one of the many timbres that Lev built into her instrument, and that was the sound that most closely resembled a human singer in the mid and high ranges.
The human ear instinctively recognizes a human voice in the instrument and for those who are sensitive to it (which not everybody is) that propels the act of listening into a strangely magical and slightly other-worldly experience.
As Chief Dan George said in LITTLE BIG MAN, “Sometimes the magic works and sometimes it does not.”