Charlie Jane Anders wrote an article at io9.com about Creating Unforgettable Supporting Characters.
I’ve included the summary list, you’ll have to go to the post to see the explanations.
- Give them at least one definition characteristic.
- Give them an origin story.
- Make sure they talk in a distinctive fashion.
- Avoid making them paragons of virtue, or authorial stand-ins.
- Anchor them to a particular place.
- Introduce them twice — the first time in the background, the second in the foreground.
- Focus on what they mean to your protagonists.
- Give them an arc — or the illusion of one.
- The more minor the character, the more caricature-like they may have to be.
- Decide which supporting characters you’ll allow to be forgettable after all.
This has obvious use with Nonplayer Characters in roleplaying games.