I've done dozens of signings, and hosted literally hundreds over the years at the bookstore I own. What goes on is usually at the discretion of the bookstore (or other event). Some of them want a reading or a talk, some just want you to sit and sign. Then for the really big names, there's usually no time for speaking because there can be hundreds of people standing in line--I'm a long way from that level, though.
I've sold anywhere from 2 books to 50 at signings (never none, which is a good thing), and of course I've seen a lot more than that sold. I always make a point of signing as much leftover stock as I can, or as the store will let me, because they'll usually put a sticker or something on it indicating that it's signed, and leave it prominently displayed post-signing, and anything that encourages people to pick up any given book over all the others in a bookstore can be helpful.