Who's seen the movie Fallen? It's about a psychopath on death row and the cop who put him there. After the psycopath was executed, his ghost flew out and into a guard. It controlled the guard until it could find another host. It kept trying to torment and kill the guy who put him in jail. The only way the ghost could die is if it ran out of breath. When it left a host, or the host died, the ghost had two minutes worth of breath to find another breathing host. If it didn't, it died.
Could this idea be used with the above? Come to think of it, I think they did that in Nomad Soul. Never really played it though.
