What I learned Friday was simple, obvious, and fascinating. If you have an interesting NPC A, you really need a secondary NPC B, that knows A somehow, for players to interact with.
Because we humans don't learn about each other through direct questioning, not really. No wonder roleplaying situations so often seem like interrogations to me and so often quickly turn confrontational.
See, we ask B about A and what we find out helps shape more subtle questions, helps us know what A is really all about. After conversing with B we are better prepared to not waste A's time, to not step on A's toes, to know best how to find out what we need to know from A. And after finding out some things you can repeat the cycle, talking to B again.
Again, simple, but I don't remember ever reading something like this. So, for every interesting NPC you want players to converse with I suggest you place a lackey, an ex-lover, a relative, a rival, someone who knows something about them nearby for players to interact with as well.
This could also have some implications for adventure design. I lucked out by having my NPCs on a ship-- basically confined in close proximity but also separated from each other in private rooms so multiple private conversations could happen. You might want to emulate that-- rooms at an inn, the separate camp-fires of a travelling caravan, etc.