If you’re a painter or sculptor, your viewer responds directly to the object you have made.
If you’re a writer, reader reaction, despite the intervention of the printed word, is still largely to your ideas as you set them down (though the degree to which that’s true is part of what drives the e-reader vs book debate).
But in the performing arts, an intermediate level of performer/interpreter profoundly influences, even determines, the experience of the audience.
A choreographer’s decisions about sound-movement-emotional coloration are made flesh by dancers, who add the complex layering, sparkle, intrigue and depth to which viewers respond. Nowhere is this truer than in a duet. Especially in works that draw on the classical ballet tradition, the emotional/aesthetic core of a piece is often rendered in dances for two.