My approach has evolved to be a bit of a hybrid...
Trap design should be internally LOGICAL but embrace FAIR PLAY. A giant slab of stone sealing off the only tomb entrance after the party is 50 feet down a hall makes sense. A real-world trap designer looking to kill could do worse. But in D&D that's rarely a good option. Instead, it could work if it adds a secondary goal of finding a new route out of the dungeon, and the DM includes a couple alternative exists. That could work.
Similarly, when you place a trap you want to think about how the inhabitants interact with it. For example, gnolls with a hyena pack aren't going to have a bunch of bear snares and pit traps lying around because their pet hyenas would be setting them off all the time. Another example, if a wizard has the 7th stair from the top trapped with a lightning bolt spewing from a gargoyle's mouth if the password isn't uttered...you'd better believe that there's some visual hint (and a pretty big one), just so the wizard and any apprentices/guests don't get zapped by accident while getting up late at night to get a glass of water.
My basic trap questions are:
- Who made it and why?
- How do current inhabitants avoid the trap? Does that make sense given how they use the space?
- Who resets it or takes care of corpses caught in it?
- If the trap recurs renewing components (e.g. poison), where do those come from?
A trap is NEVER automatically noticed due to high Perception. It's deduced by clever thinking. Look at a mimic, galeb duhr, or shrieker in the Monster Manual and you'll see a trait called False Appearance. As written, there's
absolutely no way to detect the creature's presence with skills because it's guise is just that flawless (which is different than, say, a hag's Illusory Appearance, which Insight can see through). At a
bare minimum, I expect traps to be designed such that they benefit from something like False Appearance.
Instead, certain details may be picked up (as I elaborate below), but none of those obviously give away the trap. A player needs to think a bit and interpolate to figure out the trap's trigger, mechanism of action, effect, and how it might be safely sprung, disabled, or bypassed.
You see BLANK, and here's what you observe. You encounter something interesting or unusual. What information I automatically describe may either be pre-defined (usually for smaller less-significant traps) or it may depend on other factors (for more involved "set piece" traps):
- Passive skill scores, including but in no way limited to Passive Perception. Usually I use other passive skills more than Perception.
- The presence of certain races in the party (e.g. dwarves & stonework).
- The presence of certain classes in the party (e.g. clerics and paladins & holy/unholy fonts).
- The presence of a PC of a certain background, in rare cases.
- The type of senses available to the PCs, either innately or through magic.
- Other mitigating factors.
There's usually nothing structured about it, and I do it entirely by feel and a glance at the few PC stats I have behind my screen.
If the PCs decide to investigate further, I lean a lot on auto-successes (i.e. no roll) unless there's clearly a conflict or something difficult that would draw what happens next into question.
CLICK! Now what do you do? One of the great observations that
AngryGM made is that when a trap is triggered it's best to give the player a chance to respond to it. Thinking back on my childhood, that's pretty much how I ran traps on my own because I wanted to involve my friends. Many traps are one-and-done affairs (e.g. poison needle, pit trap, sphere of annihilation in the wall); in terms of their "stage presence" they're like 1 hit point monsters. Monsters usually survive long enough to have a bit of back-and-forth with the PCs, and the 2-4 rounds of combat give the players chances to respond to monster actions. Traps are rarely designed that way, and thus don't inherently give the players a chance to respond. What I've found to be much more fun than, say, a pressure plate triggering a trap is for CLICK! the pressure plate is activated and releasing your foot will now trigger it like a Hollywood movie land mine.
Not all trap mechanisms can be tweaked to behave like the pressure plate, and sometimes once players are aware of a certain kind of trap you want encountering other versions of that trap to "remove the kid gloves". That's fine. But now I try to find ways to consciously incorporate the "CLICK rule" more often than not.
SURPRISE your players with the effects / side effects of the trap. There was a great online article written during 4e, which may or may not have been incorporated in the DMG#2 (I can't remember), which described a rolling boulder trap smashing through a wall at the end of a corridor only to reveal a room with hidden clues about the trap-maker and dungeon. That's brilliant! When I remember, I try to incorporate that kind of stuff into my traps. My players really love it, and it speaks to instigator-type players who might just trigger a trap to see what new avenue of exploration it opens up.