Building Better NPCs I: Extras, Walk-Ons, Bit Players, and Major Characters
"Not every NPC needs to be memorable. Some can be a barely-noticed presence in the PCs' lives, while others need to cast a shadow over everything the players do.
"By focusing on your NPCs' reasons for being in the game, you can determine how much effort you need to put into make each one.
"I divide my NPCs into levels based on game necessity. These levels are extras, walk-ons, bit players, and major characters."
This site has several generators that can help to give quick flavour or ideas….