Ignoring game stats, what makes for the best NPC description in a module?
Information should be presented clearly, so that when I need to check something when running the game, when I'm probably juggling six other things, I don't need to read a paragraph of text to find one key point.
The more important something is, the sooner it should be mentioned.
The more obvious something is to the PCs, the sooner it should be mentioned.
I'm interested in how we can most effectively describe NPCs in as few words as possible.
I'm a big fan of breaking things down into bullet points, and keeping those points both short and few in number. The first point should probably cover the character's expected role in the adventure, the second the key points of his appearance, and the third his motivations for why he's doing what he's doing.
Chances are some NPCs will call for more detail than a simple five-point list will allow, but if so, give the long-form description, background, etc
in addition to the summary, and give it
after the summary. That may feel like you're wasting space repeating information, but it's almost certainly helpful.
When you design your adventures, is there a "magic formula" that you turn to for listing NPC traits?
No, because different adventures have different needs - an NPC for a dungeon-crawl probably looks very different from one for a murder-mystery. However, within any single adventure you should probably stick to a common format.
What's most important to you - a description of how they look? Their motivations? A flaw? A role they play in the story? A "shtick" you can act out?
The three most important things, in order, are probably
what they're doing,
how they're trying to do it, and
why they're doing it. Then the character's physical appearance, since that's what the PC will first notice.
Beyond that, add quirks, flaws, background, etc to add as much detail to the character as you feel you need -
and no more. I'm quite happy to do a fair amount of filling in the blanks for unimportant things... and, in general, I actually prefer being left the space in which to do that.