You can't quickly create depth. Depth is the result of long-term development. What you can do is create interesting, memorable and dynamic characters who will be more likely to feature in the story long enough and prominently enough to develop depth.Your best bet to develop rich and interesting NPCs is to come up with a lot of simple "NPC seeds" who perform walk-ons throughout the game, and let your PCs choose which ones they find most engaging.

A good "NPC seed" needs:

1. A goal or purpose - Something they
   want, or a reason they do the things they do.

2. A memorable feature - I
   personally stay away from the goofy quirk stuff b/c I find it goofy, but YMMV. The important thing is not
   goofiness, but memorableness. One
   easy way to think about what kinds of things are memorable is to think about your friends, family, coworkers, etc. What sorts of things about them do you remember? Chances are, those are the kinds of traits
   your PCs will find memorable as well.

3. Something they either do to or for the PCs - A relationship, in other words. PCs are narcissists.

You can't quickly create depth. Depth is the result of long-term development. What you can do is create interesting, memorable and dynamic characters who will be more likely to feature in the story long enough and prominently enough to develop depth.Your best bet to develop rich and interesting NPCs is to come up with a lot of simple "NPC seeds" who perform walk-ons throughout the game, and let your PCs choose which ones they find most engaging.

A good "NPC seed" needs:

1. A goal or purpose - Something they want, or a reason they do the things they do.

2. A memorable feature - I personally stay away from the goofy quirk stuff b/c I find it goofy, but YMMV. The important thing is not goofiness, but memorableness. One easy way to think about what kinds of things are memorable is to think about your friends, family, coworkers, etc. What sorts of things about them do you remember? Chances are, those are the kinds of traits your PCs will find memorable as well.

3. Something they either do to or for the PCs - A relationship, in other words. PCs are narcissists.