NPCs are just tools. They're not there to steal the limelight, they're not there to give some amazing monologues. NPCs are just tools for the GM to use, to nudge the game a little bit to one direction or the other, to help make an enjoyable evening or something along those lines. As such is the case NPCs don't need complex personalities but a way to affect the PCs and through them to affect the players. When they've filled their roles, they're not needed anymore, they can be discarded, or move to the reserve pile for later scenes and/or sessions.
Drosselmeyer, in Princess Tutu understood this well. When talking to Edel in episode 12 of the first season, he says to her: "Your role is to add glow to the story in my place". Unlike the players, the GM doesn't have an avatar in the game. All the GM has is everything that is not controlled by the players. But, as we know, those things are there to serve the PCs and/or the players. The Gm should forget herself, of course, but for now please stay with me.
Most problems of using NPCs come from not acknowledging this simple idea. If the NPCs are just tools used by the GM to serve the game and players, then one wouldn’t give them amazing and way too long monologues. One wouldn't let them steal the limelight either.
But it grows deeper than that, unsurprisingly. It means that trying to give them life and complex personality is a futile intention. A gm should think in terms of how the NPC affects the PCs and through them affect the players. This calls for a simple motive, for example, so it will be easy to grasp. It also calls for something that will challenge the players' perceptions, like am NPC that divides the players and the characters to a few sides.
It also means that one shouldn't get to attached to the NPCs. Once they fulfill their role, they should finish they're part in the game. If the players don't connect to them, they should be left out. And if the players like them enough, make their part greater, make them more colorful.
So how one should use this kind of tool? There's no one true way, of course, but for me it always was about giving the illusion that they're far more complex and alive than they truly are. They should sometimes be busy, or they'll use some fancy words to describe some simple ideals. They will be distinctive, different from one another. And most importantly, they should be made such that I will always be able to bring them to the present scene, if I'll ever need them. Usually, this combination does the trick for me.
What do you think? Feel free to write in the comments.