Stacy pointed me in the direction of "Charlie Kaufman: Adventures in Portraiture" - her essay on Synecdoche, New York. It's an interesting read, and opens like so:

In Synecdoche, NY, Catherine Keener plays Adele Lack, a visual artist whom paints microscopically small pieces of art. Philip Seymour Hoffman plays Caden Cotard, a theatre director determined to express a deep guttural truth while replicating a city and scenes from his personal life for the stage.

All of these artists speak, see, and feel lines that were dreamt up by Charlie Kaufman, the writer and director of the film. But, it's not that easy.

Ignore the chronology of that last sentence, avoid putting Kaufman at the top of that food chain, include yourself as the viewer, and focus on the interchangeability.

We are all portraits and portrait artists—bodily, thoughtfully, and sleepily while sharing an identity of fiction. (Source)