Giving your NPCs heart and soul.
focuses on techniques that give major NPCs emotional depth and complexity.
They say children usually drown in the shallow end and I believe it. The word “shallow” itself has a cold breeze blowing through it, as in “shallow grave.” Try diving into the shallow end of a pool, and there's nowhere to go. All that comes of it is an opportunity to get to know your local head-trauma specialist on a first-name basis.
For either major or minor NPCs, it's not a requirement that they have emotional depth. If your game is comedic in tone, you might not have a single NPC with depth. This would be completely fine.
That having been said, you'll ...