The techniques presented here to create viable and artful Character Diamonds and make NPCs interesting just scratch the surface. Going into all their facets would add far too many pages and be too specialized, but here are a few last points to consider:
You should give thought as to whether or not you want any of your NPCs to have Traits in common.
There are matters of false fronts. Sometimes an NPC will have one Trait, such as arrogance, that covers up another Trait, such as insecurity. I call these false fronts "Masks."
Sometimes an NPC might struggle against one or more of his Traits. For instance, a solider might be a coward but struggle to be brave. This takes fairly sophisticated writing to communicate artfully.
Character Diamonds, or combining even a couple of interesting Traits, are useful techniques for creating interesting villains. Other methods besides Diamonds, however, can make villains more dimensional and lifelike, and therefore emotionally engaging. You can humanize them by seeing the fears or emotional wounds that drive them. You can reveal tiny pieces of their lives outside their acts of villainy. You can clue us in to their motivations, which, even though they're heinous sounding to us, make sense to them.
But don't humanize a villain too much or the player will feel guilty for killing him or her, which in most cases is undesirable. On the other hand, if your villain is to change sides at some point and become good, then perhaps giving him or her more "humanity" in the beginning will make the player both be reluctant to kill the villain, and be glad when the villain changes sides.
Traits and quirks can act in harmony or can fight a bit with each other. If they fight too much, the character will be unfocused. For example: A character who likes to have things "organized" (a Trait), always keeps his car messy (a quirk). These two seem to fight each other and will blur our sense of who the character is.
If the character likes to keep things organized, but misplaces his keys on a regular basis, that doesn't fight his Trait of "being organized" too much and would probably work although there's still the possibility that you'll confuse the player as to who the character is.
Usually, quirks don't fight or harmonize with a Trait. They simply add more detail to the character, as in a character who is always organized (a Trait) and who enjoys watching baseball games (a quirk).
There's always something more to learn in this, or in any other art form, but if you keep these Character Diamond techniques in mind, you should have a solid basis from which to begin.