film industry - What factors go into determining what trailers are played before movies?

I have a friend who owns a (very) small theater... so some of this probably isn't "official" (I know they bend the rules a bit at times). Also, they operate only on film, I'm not for sure about digital

Anyway, one thing a lot of people don't realize is that film is usually shipped on 6 or so small reels. The movie theater then tapes together these reels into usually 1 or 2 big reels(depending on what's capable of the projector and the size of the movie). During this time, they add on small preview reels.

From what I can tell, usually 2 or 3 previews are shipped with a particular movie. It's not mandatory that theaters put them on, but they are usually the latest, so it gives theaters a good incentive. Unlike movie reels which must be returned, preview reels can be kept by the movie theater. So the movie theater can of course add on more previews if they wish as well. I believe it's allowed for them to add no previews on as well.

Of course, with digital there is much less choice. I believe the previews with digital movies are completely fixed.. So, it's completely up to the studios..

As for how they select which previews are shipped with blockbuster movies, see the other answers