Well, I've never been on a porn set, but I have seen photos from behind the scenes. It struck me as being very much like a non-porno movie or TV-show set. A LOT of people behind the scenes. You have to be willing to fuck in front of quite a crowd!
Yes, many times. It's all been explained before. It's just like any other movie set, except the actors are naked and fucking. If you've seen one movie set you've seen them all.
Never officially but I swear my Uni Dorm room might as well have been a porn set that first year - we all "entertained" and no-one ever batted an eyelid.
There was that odd fapping noise in the background though, faulty fan