The whole thing about posessions is rooted in fear and with very little substance, mostly the stuff of religious hysteria I guess.
I remember clearly reading comments made by Ted Andrews( Look him up ). He said that for all the times he's been called out to help someone who's reportedly been possessed by some evil spirit, he's yet to encounter an actual possession. He said possessions simply don't happen the way Hollywood or some of the more sensationalist authors make out they do.
A possession can only really happen when someone wants it to happen to them and goes some to make it happen. We're much stronger in this world than spirits, and for any spirit to take control of us in any way, bad spirits or good, they must have our consent. It is really really RARE for it to happen on it's own, unless an undue a mount of fear of possession is instilled in the person.
Exorcisms are in my opinion not the best way to deal with stuff like that, there are better ways than fighting the energy.
Dont get me wrong Sebastian ;)