Saturday night, my party had fought their way through a few mutant experiments, and were understandably on edge due to a number of clues and hints about an odorless, undefined gas that may or may not have had negative consequences. I had been building up the clues all night long; they had found the dead doctor in a corridor, they had located his syringes and tools, and they had killed what they believed were his subjects.
You can feel the tension rise on a night like that. All it takes are a few obscure references to something concerning or uncertain. The usual ones are, of course, 'People go and don't come back' or 'No one knows what's past the [insert feature here].'
I have found, however, that anything not fully explained will usually fit the bill. A torn letter with a few key words ... those that people are uncomfortable with. Anything scientific or associated with the human body. Anything overtly occultish. One small reference to a removed liver or a gateway that can't be closed to ...
Works every time.
So when they got to the locked door near the end of the night, and discovered that the lock was fixed to a trap, they were good and uncomfortable. They descended the last sixty feet in an odorless thick fog that was definitely not water and definitely not smoke ... but retained the features of both. I have already been playing with rules for awhile now that thieves can be certain of opening a lock, given enough time. I explained how the lock was hooked to a cord that extended through the door. The 5th level thief could either try to remove trap, or he could try to open the lock without setting off the trap.
The way that worked, I explained, was that IF he could undo the lock quickly, there'd be no problem. The number of rounds it would take would be 3d4 minus his level (slightly different from the post). If he tried that, I'd determine afterwards whether it was quick enough.
He decided to try removing the trap. His ability was 45%; he rolled a 58 and failed ... but didn't set off the trap.
So he tried the other way. He rolled a 9 on 3d4. Minus his level, that made a 4. I then explained that if I rolled a '1' on any of three 4-sided dice, one representing each round before he got the door open, the trap would go off. At first I was going to throw all three dice ... but my instincts kicked in. I picked out one and started shaking it.
Now we come to the point of this post. So long as I had that first four-sided rolling around in my hand, I owned that player's ass ... all the players, really, since they all depended on the trap not going off. The last thing I wanted to do was to throw that die right away. The thing to do is milk it, rattle the die around in your hand for a bit while you explain something else, anything else - where the party is in relationship to the door, the sweat on the thief's brow, the amount of time left on the party's torches.
When you let that die go, the last thing you want is for it to come up a '1' ... because you haven't lost anything if it comes up a '3' or a '4.' You still have two more dice to go.
As you're shaking the second dice, the party will start to do your work for you. They can see you're milking it - they know you're fucking with them, and they'll comment and joke about it. That doesn't matter ... because a part of them doesn't want to see the die hit the table at all. So long as its in the air, there's a chance. So the more they point it out, the longer you can milk it ... no matter how genre savvy your party gets, the game is STILL in that die that hasn't been thrown. And when it is, damn, hope its still good for the party.
You let it drop and again, the trap doesn't go. You pick up the third die.
Now you can talk about anything. The party has had relief come twice; they know the odds are in their favor now. The tension is at a peak. If the die hits the table good, there's going to be cheers and relief; if it hits bad, there's going to be fury and recriminations. You can already hear the party starting to prepare themselves for the worst.
Meanwhile, you just keep shaking that die. You talk about anything else. Talk about what you plan to do the next day. Ask how one of the player's sister is doing - if she got the job, whatever. Drive the party freaking nuts, but don't roll that die until they are fucking screaming at you to do it ... because it isn't until then that the tension around your table matches the tension the actual characters would be feeling in the actual situation. When you have that parallel, you can throw the die.
I don't know exactly how I learned to work this out as a DM. Seems to me it was always there to some degree, even in the beginning, when I was just a kid. A lot of DMing is fucking with people's heads, in a very particular way. It's recognizing that when you stand up at the table to watch someone throw a die, that sends a definite message that THIS is something really important. It's not that you should pretend to do so ... is that when the die actually is, you should demonstrate a body language that suggests it. Your gestures demonstrate a great deal. If you're relaxed, laid back, unconcerned ... this will produce a particular result. If you roll a die and make no reaction, because you don't care, expect little empathy from the party. But if you roll a die that sincerely bugs you, that sincerely does not fit with what you had hoped for (that you'll get to roll all three dice rather than just one, for instance), then get mad. Don't explain why you are, of course ... the less the party knows, the less comfortable they are going to be.
I don't recommend inventing emotions and play-acting ... it will be insincere, it will be recognized as such, and you'll do it at the wrong times. Random emotion will ruin those moments of true emotion ... which should generate naturally if you, dear gentle DM, are adamantly involved in your game. You will feel frustration; you will find amusement in things; don't slap on a death mask to cover those moments. Feel them. Let the party see your responses.
There's NOTHING more disquieting than a DM that can't stop giggling - for no apparent reason - as you're trying to descend into the Pit of Despair. Parties tell me they hate it when I start swearing and reaching for books out of the blue. What the fuck have they done? What's going to happen? What's so goddamn funny?
And don't roll that critical die of certain death until you have to. Hold that puppy, get on your feet and lean out towards the party and say, "God, I hope this doesn't kill somebody." Then complain that you don't want to throw it because you don't want anyone freaking out. Patiently explain that it's just a game, and that people can roll up a new character. Tell the party exactly what number it is you don't want to come up. Then put your hand over your eyes and throw.
You can tell from their reaction if it was good or not.