Your players won't remember the perfectly balanced encounter. They'll remember the NPC who made them laugh, think, or rage.
The 3-Tier NPC Method
Tier 1: Walk-Ons (60 seconds) - One memorable detail. One sentence of motivation.
Tier 2: Recurring (5 minutes) - Name, role, 2 traits, one secret, one relationship, one want.
Tier 3: Pillars (10 minutes) - Full profile, backstory, goals, relationship web, voice notes.
The Promotion System
Start every NPC at Tier 1. If players engage, promote to Tier 2. If central, promote to Tier 3. Never over-prep NPCs players might ignore.