Who is the narrator, can she or he read minds, and, more importantly, can we trust her or him?
N/A—It's a drama
You can't really talk about a narrator per se with dramas, since the characters do all of their own talking—there's no outside narrator shaping what's going on. That said, factors such as the play's structure and stage directions can frame the drama in much the same way that a narrator does.
In this case, from Shepard's stage directions, we can tell that he's trying to present a very drab and depressing atmosphere—this is not a happy place, and from the minute we read about the set decorations (or see them in a live production), we're not expecting the good times to roll. Shepard doesn't use stage directions to editorialize too very much but they do suggest Shepard's "slant" on the drama he presents.