In our book Character Foundry, we set out the concept of “Great Material”. Surround yourself with great ideas and they will spark ideas within you. Further, you can take bits and pieces from other things and incorporate them, making your job as GM easier and often times better. An example was using the annoying friend from an old sitcom and marrying him to the real estate broker from a current movie to make an NPC couple you need in a city. Poof - You know what they look like, sound like, and probably how they will act in many situations. But when do you do this? You do it for the really big games.
Take our merchant war. There are so many characters buzzing around this thing, many of which have never entered into play before. How does a GM intelligently guide the NPC actions and reactions? By knowing the characters. But you can’t do that (at least not easily) for three dozen made from scratch NPCs. Giving some of these new NPCs personalities from movie characters, I can far more easily think, “What would Draco Malfoy do?” This does a couple of things - Mainly, it gets me there quicker. Also vital - It stops everything from being logical. What a boring and easily defeated world you would have if all of your NPCs were logical. Sure, the smart move might be to forgive and forget, but if the NPC is brutal and vindictive, that ain’t gonna happen! Maybe some NPCs are cowards and even though they have the might to force their will, they might be afraid to risk it. Or the reverse might be true, and the character might be the world’s greatest bluffer and his assumed might is really all illusion (maybe literally).
Personalities aren’t that important for NPCs you expect to be killed before they utter a sound, but for the big games, especially the ones that don’t involve a “dungeon”, you need to know more about the bad guys!