Yes. I believe I've seen it, pretty obviously...even had a few battles with my own, but I don't know if that is officially possession....you can also be oppressed by them.

I think possession entails being taken completely over by them at times.

However, there is also the possibility of having multiple personalities.

But in my experience, demons are quite non-human, and stand the hair up on the back of your neck if you encounter them.

Then there is also the theory of sharing your body with someone else's spirit, but I don't know about that one.

There is no scientific way to prove any of it one way or the other, so it is all subject to individual interpretation.

But I don't let stuffy theologists spoil my interpretations with their scientific knowledge. That is not the end-all of it all.

...just had a thought. What if the deep desire to die is a torchered spirit who perhaps died suddenly or something and is taking over the body to try and live out the process of death or something. A bit of a strange thought but thought I'd put it out there.

nope.

Oh totally, the Exorcist story happens all over the world every day. I'm catholic so I do.

A few months ago I would say yes, but right now I am thinking that all of my touches with the afterlife and god might just be me losing more of my mind.