What are the general characteristics of vampires?
Vampires look like normal humans until the feed is on them. Then their foreheads change, their eyes turn yellow, and their teeth become pointy with extended incisors. (see Welcome to the Hellmouth.) They are demons who possess a person’s body after death. In The Harvest, Giles says, You are not looking at your friend, but the thing that killed him. Vampires also seem to achieve their Game Faces when they are emotionally stirred (Angel in Angel) or among friends (all vampires around the Master in the first season, except for the Anointed One, Angel in School Hard). Whether this particular feature is physiologic or demonic in nature is subject to heavy debate.
