Let’s start with something small. See what I did there?.. Never mind.
This idea isn’t terribly original. Something similar was in DMG 2 (4th edition), and probably in other places as well. But it works surprisingly well, so, hey, why not. Sometimes a bunch of NPCs need to talk to one another, and it would be nice if the players could witness it. A war council meeting, an inquisitorial trial, a courtly intrigue, whatever. Sometimes PCs can participate in it, often they’re not important enough to contribute, or simply aren’t there. But watching a GM talk to himself for half an hour is boring, and gives said GM headache.
Solution? Simple. Let the players do it for you. Print out brief character descriptions for each participating NPC, anything from a single sentence to a paragraph. Who they are, what they want, how they’re going about it. Simple conflicting motivations. Add a picture if you’ve got one. And distribute them amongst the players however you see fit. I tend to leave a tie-breaker NPC to myself, in order to help steer the discussion and the overall plot direction. Presto! You’ve got your vignette.
Let it play out for a while, for everyone to introduce themselves, state their positions and argue their cause. Sometimes, players will surprise you with their interpretation of your NPCs. Which is of course fine, as long as they’re not obviously exploiting the situation in favour of PCs.
It probably shouldn’t last too long, as the game is about PCs, after all. Which is where the tie-breaker comes into play. As soon as the players start getting bored and the discussion starts to circle, it’s time to wrap it up. But that’s true of any discussion.
One thing I want to try out with this setup, is have enemy minions report on PCs to the Big Bad, perhaps as often as after every adventure. And let the players define more and more about the mysterious villain and his connection to them, without even realising it.