I ad-lib any and all social encounters, because I have no idea what my players will do. I might make some notes to determine the NPC's interesting characteristics, but for the most part, I let it all hang out, and go with the flow. I try to put myself in the NPC's shoes, and think about what I would do in the situation.
Also, unless I feel the being has a very good reasons to attack the PCs, monsters rarely attack the PCs right off, unless threatened first. MAkes for some interesting encounters. (Like the Otyugh the players befriended a while back. HEy, the otyugh had plenty to eat, and it was bored. The players started talking to it, so it replied. Even helped them out in a couple fights after that.)