Mysteries: A Golden Rule

One of the classic problems with mysteries in D&D was the Paladin's ability to detect evil at will - he could just scan everyone, identify the one person who pings as evil, and that's your bad guy. (I exaggerate, but only a little.) The other classic problem that I've seen when people construct mystery scenarios is that you tend to have the Big Bad and a bunch of other NPCs. And because those NPCs are not the Big Bad they're always open and helpful - and so identifying the one guy with something to hide becomes easy.

Well, there's a shock.

Of course, if you actually read some mysteries, especially by people who write them repeatedly and well, you find that it doesn't work like that. Instead, the protagonist meets resistance on several fronts, as people simply refuse to speak to him, or they're evasive on particular subjects, or they warn him off of investigating some topics (which, of course, encourages the protagonist to investigate exactly that).

And the reason for this is that there's a golden rule for writing mysteries: everybody has something to hide.

What that means is that every character should have some subjects they're just not willing to discuss, every character should have some things they're willing to lie about, and every character should have some secret that they don't want to be revealed. The PCs' job, then, is to tease out the truth from the set of inconsistencies - if A say this and B says that and the physical evidence says this other thing, then the truth must be...

(Once that's established, it becomes a matter of applying the Three Clue Rule - for every conclusion that you want the PCs to reach, you should provide three big clues. That way, they'll miss one, misinterpret a second, and get it on the third.)

It's also worth noting that all those lies and secrets should not (or at least should not necessarily) make the person doing the lying a villain - a person might well have made a stupid but innocent mistake that they're now hiding.

Of course, for extra credit you could set it up such that the person is willing to reveal the truth, but only in the right circumstances. For instance, a man might not be willing to discuss his whereabouts, and therefore his alibi, in front of his girlfriend if his whereabouts were "buying an engagement ring for her" - but might well be happy to discuss that when she's not around. (You'll note that that is also a secret that doesn't make him a villain - see above.)

As regards the problems created by detect magic, these are usually caused by people forgetting that the PHB specifies that humans don't have a tendency for any particular alignment, including Neutral. Therefore in any settlement fully a third of everyone the PCs meet should ping the Paladin's spell.

(And, yes, it's also the case that in addition to the 'main' evil that the PCs are confronting you'd also expect them to happen upon a whole load of other nastinesses that they might or might not want to clear up as well. So maybe that creepy old man isn't the murderer, but if he's instead using his business contacts as a front for drug smuggling, maybe the PCs want to stop that as well?)

Anyway, that's that. To recap:

• When writing a mystery, be sure to include plenty of people who might be the villain.
  
• Every major character should have topics they don't want to talk about and secrets they don't want shared. (They might be willing to talk about some or all of these under particular conditions.)
  
• Those secrets don't necessarily make the person a villain, and even if the person is a villain it doesn't necessarily mean they're the villain.
  
• In the course of the investigation the PCs will likely uncover a bunch of other issues. They may or may not want to fix those at the same time.