In a G+ post yesterday Zak asked "What do you know about an NPC before they come up in your game?" - a question that I think is really interesting. I think that over the last year, which is as long as I have run games, my approach has changed a little.
The first game that I GMed was In A Wicked Age - by design, I knew nothing about any of the characters before we played. NPCs were revealed as players asked questions, and were sketched from stick figures into something resembling real people over the course of the first scene that they were in.
Dogs in the Vineyard, which I ran as my first short campaign, was more interesting. As NPC stats are descriptive by nature, this plants seeds in my mind about what the people look like. How they walk and talk maybe. For some of the characters they look like people that I know or have seen on the TV. Some - typically the people who are props perhaps - are less fleshed out. As the campaign wore on the potential for some NPCs changed a lot.
The hedge-witch/trader that the Dogs were tailing turned up dead, badly mutilated. Now, originally this was going to be the work of the mad-with-grief Dog-turned-Steward in the town. But... As they investigated this didn't sit right. And so the dabbling-with-demons brother and sister became the "oh King of Life make it stop, they morph into Resident Evil style monsters and they just keep coming" finale bosses. And their grandfather was supposed to be dead, but good things come in threes...
In LotFP/Somewhere North I have used a few generators just to generate details about people. I have stats and nothing else, or a one-liner (a retired hero lives there; a war criminal is in hiding) and this becomes the basis for something else. Names are good. I can't put my finger on what it is about the names that lead to descriptions, but they do something. Randomly rolling Ungrall the Unctuous instantly put the person in my head, with all but one detail missing.
A detail that is always missing for me until the NPC appears on-stage. The voice. I never know what a NPC will sound like until they say something. Never. Zinternik's elves and halflings are all cockney. Ungrall the Unctuous speaks with a pronounced lisp due to tongue boils. Robin, the head guard of Overtornea is incredibly officious and bored. Marco the magic-user is incredibly excited and casts spells like he is throwing Pokeballs.
But until they open their mouth for the first time, even I have no idea what they will sound like.