Now, I don't know what sort of games you run, but I tend to keep the number of NPCs to a minimum - for a number of reasons:

1. If the players know a lot of people in the city they tend to spread up often and run about their own business, talking to their separate contacts, which gives the game a rather fragmented atmosphere, and you often end up being the limiting ST, stating "you're not here right now, so you can't partake in the conversation!".

2. Even the most skilled actor can only come up with a limited amount of accents and styles of expression, so you...