demons

You will, so long as you are willing to put in the effort. I might try to converse with daemons again, but I get the feeling there isn't much more I could find out with my current line of questioning. If I tried to contact one again, it would probably be to make some kind of trade for information.

what do you mean trade for information?

by the way isn't possession just a Hollywood gimmick I thought only one "soul" could harbor one vessel

Many daemons are hundreds of years old and have been summoned many times over their lives, probably by powerful and knowledgable magic users. I figure I might be able to find some lost knowledge by interviewing daemons. They also have more experience with magic than most humans, so it just makes sense to me to use them as a source of info. However, I expect to pay a price for this knowledge, precisely what sort I cannot be certainly for that reason I inted to pursue knowledge elsewhere first. I think possession does happen, but I am certain of very little besides that.

I've talked to "demons" (mainstream crap), on a number of occasions, for a number of reasons.

Possession, is what you make of it, in my opinion. I do it voluntarily, to reach a higher state of consciousness.
As for television and movies, I can't comment. I see it as crap, with no clear proof of possession...

I always set up "gifts", pertaining to the type of summoning, as trade for the information I seek. If accepted, and I show respect, I usually gain the knowledge I seek.