Quirky is cool and fun, but it only works if there is a reason. You don't have to get into a psycho-analytical involvement, but it helps maintain the continuity if there is a reason for the quirkiness. In my newbie game there was a small mage guild in town. The 'host' mage (think greeter at wal-mart) was a very difficult man. He asked a lot of questions and when answers were given he became very suspicious. He would do this for a while until it seemed the business was legitimate. He figures if he acts crazy enough people won't bother him and the others for help as much. Otherwise villagers would be bugging them constantly. The players never knew why he did what he did, but they still talk about him today. (Partially because I was fucking funny when I played him.)
There are some quirky NPCs that don't need that much detail. Maybe just one thing that sets them off. A personality trait or physical tic or feature that's exaggerated. All it takes it one memorable detail and the players are more likely to remember. Ex. Herbert the merchant deals in fine wines. Or. Herbert the Merchant is known for his fine wines and giggles like a small girl when he's amused. Not the best example, but now the players have an identifier hung on old Herbert making it easier for them to remember. "Lets go see the giggle guy for some wine."
Don't mistake quirky characters as secondary, or weak or bit players. They can be significant and have a great deal of influence in the game. And a quirk does not need to define the NPC, but accent him or her. I'll use a Whisk analogy, the quirk isn't the meat, its the seasoning. But you have to make sure you have a good piece of meat of the seasoning won't matter. (I learned this trick over at Hack & Slash, Thanks Courtney)
Like all good things, don't overdo it or the quirkiness becomes watered down and becomes a David Lynch movie. Oww, didn't see that shot towards David coming. But being weird to be weird is boring and not weird.
I'm done. Off to work. Hi ho. Hi ho.
