1. It doesn't matter. If we sanctify ourselves from those worldly things in the first place, then it literally doesn't matter.
2. The Bible never gives us indication that devils are allowed to take on the form of a person, although, the absence of it doesn't eliminate the possibility. However, what's the point in possessing a person if they can just become one? You can do a whole lot more damage as one than just possessing one, which leaves us to deduce that God does not allow it based on the lack of Scriptural basis for it.
3. David Blaine first started to get popular about 25 years ago, and he did card magic tricks on the street. I video taped it when I was a teenager, and watched closely. I was able to replicate some of his tricks; I figured them out. They were much more simple than most people realized. There are tons of illusions that are done by simple methods and sleight-of-hand which most people don't realize how very simple it really is.
4. I was asked this many years ago about someone named Chris Angel, a co-worker asked me to investigate him. Turns out it was really easy: I just read the fine print on his website. It stated very clearly that he uses camera tricks for entertainment purposes only.
I'm not saying it's wrong to watch a video, but when simple matters of sleight-of-hand become questions of whether or not the person you're looking at is an actual devil, I think it might be time to stop watching them.
