Trope, Flaw, Skill: 3-Layered NPCs
DIY and Dragons's Landmark, Hidden, Secret is nowadays an iconic part of location design for adventures. However, this philosophy can be applied to NPCs as well.
Upon first meeting an NPC, the players immediately discover its Trope (the Landmark, in this analogy), a cliché the NPC is based on. The trope-iness here is important, as the players need something to latch onto and understand easily.
If the party spends some time with the NPC beyond the first meeting, they'll figure out their Flaw (the Hidden information, in this analogy). In my games, the most common flaw is a vice, but it can also be something relating to their past, or a personality trait.
Once they discover the Flaw, start dropping subtle hints to the NPC's Skill, which is something special they have or can do unique to them. To discover the Skill, they might just connect the dots based on the hints (like an NPC whose skill is magic-detecting sight might never have been it by a spell), or they might speak to the NPC's connections, or go on an adventure with them, or in general anything that (by your metric) constitutes meaningful interaction beyond just investing time. The Skill might be an actual ability, a connection to somebody important, a piece of equipment, or some rare knowledge.
Example NPC generator (d6 on all columns)
| # | Character | Flaw | Skill |
|---|---|---|---|
| 1 | Lazy Town Guard | Drinking | Immune to the Plague |
| 2 | Honourable Knight | Gambling | Inspiring Speeches |
| 3 | Menacing Bandit | Obsessed with Money | Knows the Land |
| 4 | Boorish Drunkard | Bounty on their Head | Has a Secret Hideout |
| 5 | Fat Monk | Flat Broke | Slips Unnoticed |
| 6 | Bespectacled scholar | Compulsive Liar | Expert Occultist |