I will usually say hello when I happen to have a next-seat neighbour, and almost always immerse in either IFE or my own entertainment - music and/or book -, especially if in a long haul. A couple of times I've had wonderful conversations with seat mates, but I do remember that one time when I had an empty seat next to me and before I could claim it the most obnoxious person sat down there and proceeded to talk for almost the whole duration of the flight - about 12 hours - without getting any hints. That was indeed painful.