I know I see faces in dreams because they're often people I know, occasionally I've seen a celebrity I recognized.
I had a whole series of dreams several years ago, some of which involved reptilian aliens (which I had never heard of before, but have seen drawings of in UFO literature since, looking exactly the way I dreamed them) and several involved long interactions with hordes of the undead - but not like zombies, these corpses were quite animated and talkative, just dead and decomposing. I once had a long conversation with an attractive dead girl on a subway train.