Every once in a while, a "big" play like Angels in America comes along, but many (if not most) of today's playwrights, and particularly the less experienced ones who watch too much television, are writing smaller and smaller plays. Too many plays have become insular, relationship-centered affairs. To be honest, how many times can we watch a few characters going back and forth? No matter how witty the dialogue or the twists and turns of the plot, sooner or later we run into the limits of the play, and we're bored.
That's where the absent character comes in. Perhaps the most famous example is Beckett's Waiting for Godot. Two men wait on a road for Godot to show up.
So the next time you're feeling a little claustrophobic during playwriting, throw in an absent character and feel your play expand.