Obviously, it depends on exactly how important or present the particular NPC is going to be, but I tend to be very fast and loose with assigning NPC numbers unless I know they are going to play some significant role in the upcoming adventures.
For example, if the PCs end up in a situation where I suddenly need stats for the town guard, I just pick some numbers that sound appropriate and go with it.
Jim-Bob, Town Guardsman: Attack +3 (longsword, 1d8+1), AC 14, Hit Points 10
This process takes me about 10 seconds. I just pick and choose and don't worry about the details like strict BAB, Save and Skill Point balances. In most situations, that's generally all I need. Now, if the PCs begin showing interest in interact with Jim-Bob beyond a standard "help him/attack him" capacity, then I'll start to fill out other things as need.
Jim-Bob's surname is "Hunting". He is married to seamstress named Emaline. They have 2 sons and a daughter, all under the age of 10. They are devout church-goers and often spend time at Emaline's family cottage in the hill country a half-day's ride from town.
This extra detail will take a couple mintues to fully solidify, one piece developing at a time. Where Jim-Bob develops from there is based upon what the campaign and PCs demand of him.
Generally speaking, I let common sense prevail and fill in the blanks when they are needed and I try to avoid needless rules detail. Afterall, in all probability, the PCs would never know if I gave Jim-Bob 3 ranks of Appraise, so I won't spend that effort unless I think it is needed.
HARRY DRESDEN — WIZARD
Lost items found. Paranormal Investigations.
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