One of the things I did at GenCon was attend several seminars from the Writer's Symposium. I did it for two reasons. The first, as my bio says, is because I have interest in writing and would like to be doing it professionally some day (though, as of yet I'm still writing trunk novels.) The second is because there is a lot of writing advice that is also applicable to gaming, especially GMing. One of the tips I picked up, a skill called Blitzkrieg Characterization is one of those things.
How It Works
Blitzkrieg Characterization is a way to quickly get people to start processing a character while simultaneously adding some depth. It works by presenting a seeming paradox about the character, a puzzle that has to be worked out. Done right it gives a glimpse to a character being much more than what they may first seem. Done wrong...well, your players will just get to figure out your tropes.
Two Things For, One Against
How you do Blitzkrieg Characterization is essentially a variation of the classic "define three things" technique used for making NPCs quickly. You're still defining 3 things about the character, but this time when you define the third thing you are making it a fact that goes against the grain of the other two.
Bill is a pig farmer. He spends his evenings down at the bar drinking beer with his friends. He never misses a show at the Theater.
The first two are in agreement with each other. It gives an image of a rough and tumble pig farmer that loves his beer. The second however adds contrast to the character. There is a level of civilization to the character, and that has to be accounted for in the player's minds.
A more game related example would be what a friend of mine did with Hida Sousuke.
Hida Sousuke is a hida bushi and chui of the legion. He is large, broad shouldered, and strong as an ox. He drinks his tea out of tiny, delicate tea cups.
Things To Watch For
You want to be careful when doing this that you don't always stray to the "opposite" in the same direction. In the above examples I contrasted both Bill and Sousuke with "classic" (or stereotypical) examples of things people of 'culture' do. If you always do that then you end up in the position I mentioned before with going into tropes. So try to mix it up.
Perhaps Hida Sousuke keeps a finely detailed ledger of all his expenses. Perhaps, despite his size and brawn, he is a knife fighter or a ninja. Whatever way you do it, just make sure it contrasts. It will help your players make the NPC stand out, and help you make more three dimensional characters.