The idea of the alien is: Start as a regular person. Every night, you must swap bodies to another person. When you do this, you (the alien) burst out of your current body and climb into a new one. You would then become the new person, including their name and color. They(the player) would obviously die, due to their body being taken over, but nobody would know.
Perhaps a way to kill the alien would be: if the doctor heals someone while the alien attacks that person, the alien dies to the doctor. At this part you can have a zergling death animation or something.
Good idea?