I was curious to hear people's opinion's on this but I'm not even sure how to answer that one
I know some people who practice a certain ma who involve certain rituals in their training from time to time.
In which they call upon a spirit for example a deceased teacher to enter their body.
That way the spirit or soul or what you want to call it from the deceased teacher will take control over the body and move in the same way he would have done if he was alive.
You can tell the difference when a body moves like that if it is possessed or not.
(everybody moves different even in the same style).
I've heard this from several people around me, all very credible. And I know someone who participate in this. Although I've never seen it myself.
Also this would come with a price to pay though.
But these "spirits" don't accept money. So if you do this you will probably grow old a lonesome man, some of your closed loved ones will fall terminally ill or get in to an accident or something along the lines of that.
They still practice this in parts of Indonesia.
But on demon possessions, I wouldn't really know.