I stumbled upon the DNA method, posited by a user from Montreal named Acer on the roludo.ca forums. (Links: thread one; thread two.) To differentiate an NPC, simply define three elements, or aspects:
- Demeanor (the face they put on for others to see)
- Nature (who they are at heart, or in private)
- Agenda (what they want in general, or specifically from the PCs)
Obviously, this can be laid over any existing NPC framework. For DitV, I would roll up a sheet of NPCs as the rules call for, then add the DNA just before or even during a session of play.