Writing for people not for instruments

Recently it was suggested that I write a series of experiential pieces (see past posts) that were less general. In response I've begun writing a number of these pieces for individuals. This has caused me to think more about how I composer for performers and I thought it might be interesting to think through that process here, maybe you'll find it equally interesting.

Music for Children is a piece I'm writing for saxophonist Sophie Gibbett. Originally inspired by Sophie's wonderful tone in her playing, the piece has quickly become less about the instrument and more about the performer. Music for Children is now a piece that explores playfulness and childlike-ness (in the positive sense) and the interactions between child and authority. This change took place as I became less interested in the instrument, and more interested in the person performing it. 

Following this I have started to collaborate with Kira Thomas on a piece that aims to bring Goth sub-culture into the western classical performance space. While in its early stages this work has begun from an interest in the performer as opposed to the instrument.

One question this has raised is of multiple performances. Can a piece written for a performer ever be performed by another person? If it is, does it remain the same piece? Perhaps someone other that Sophie can perform Music for Children but in doing so they become an avatar for the original Sophie. I don't know the answer to these questions, I may never know. I'd be interested to hear your thoughts.

What I do know: right now I'm finding it far more interesting to write for a person rather than an instrument.