In ancient Greece, actors wore masks. They showed emotions not through subtleties in facial expressions but through words and the emotions of the body. The word prosopon is the word used for mask - "in front of a face."
That is the term used by the early church to talk about the Trinity, the "three persons." The notion that a prosopon, the thing in front of the face, is a person doesn't show up until several centuries into Christendom.
This nuances the way we talk about personhood or selfhood. It has its origin in the mask that was placed in front of an actor in a drama.