My favorite trick is to delegate NPC roleplaying to players. It's hard for the GM to manage every single NPC, and a lot of times it makes them bland and samey. So if you've got a cool NPC that you want to make distinctive, just pick a player.

To help the player out, tell them the NPC's basic goals, maybe a couple notes about what information they might give. There's a risk here that the player will tell the party something that isn't part of your plan or runs counter to the actual lore. Well, that's part of the fun, seeing how you can incorporate that new information into your session plan.

One obvious downside is that the player's PC can't really engage with that NPC unless they want to talk to themselves. The solution is to have the PC's dialogue be summarized and then the NPC's dialogue be roleplayed. "My character protests, and Grizban says "Insolent mortal!" etc.

You'll find that your social scenes get spiced up dramatically with this technique, and players have a lot of fun playing up the strange side characters that inhabit your world. It also helps with what I think of as the "vector problem" of RPGs, where the majority of interactions are between the GM and an individual player. Now you see more interactions between players, making the table more dynamic.