What is suspense? That's the question I'll be looking at today. Specifically we'll cover the role of the following in creating suspense:
- Dramatic irony
- Well-defined stakes
- A ticking clock
What Is Suspense?
Suspense is an escalating sense of apprehension or fear, a building of pressure, heading either towards an uncertain conclusion or a horrifyingly certain one. You (/the reader/audience) might know there's a giant monster at the end of the tunnel and our heroes are heading toward it. Or it's a ticking clock of what-the-hell's-going-to-happen and we don't know what's coming. Suspense is getting your readers to ask: What's going to happen next? 
So we need:
a) A real danger to the hero and
b) the possibility that the hero will escape the danger.
c) A finite amount of time (/a ticking clock)
Further, dramatic irony can be used to increase the audience's sense of curiosity and concern for the hero.
Dramatic Irony & Suspense
Scenario 1: Imagine a hero inching along a darkened path, oblivious to the deathly shadow soundlessly creeping up behind him, poised to suck the lifeforce from his bones.
Scenario 2: Imagine that, as before, our hero inches along a darkened path anticipating a threat just round the bend. He doesn't know whether there's a monster there, but there could be. Unlike before there's no deadly shadow stalking him ... at least, not that we know of.
The first scenario creates suspense, in part, by giving the reader/audience more information than the hero possesses. We see the danger creeping up on him and want to scream: Turn around!
In the second scenario there is no such disparity of knowledge. We know what the hero knows and, with him, we cringe as he rounds every corner, every bend in the twisty road.
The elements of dramatic irony:
In order for dramatic irony to exist there needs to be a difference in how much two characters, or a character and the audience, know. Generally speaking, there are two possibilities:
1. The audience knows more about the danger than the hero.
This is what was used to generate suspense in Scenario 1, above. You, the reader, know there's a monster lurking around the next bend but the hero doesn't. Perhaps the hero thinks the evil has been neutralized or he thinks it's somewhere else. But it isn't. It's lying in wait for him and as soon as he rounds the next bend it's going to attack.
Or perhaps the villain has set a trap for the hero that the hero is oblivious to. The hero is rushing headlong to help someone in need. We see the villain set a trap for the hero and watch as he runs toward the trap. We want to warn him, to shout out that it's a trap, to stop, to go another way, but the hero keeps running and we are helpless to prevent the outcome.
Or something like that.
2. The audience knows less about the danger than the hero.
There is a scene in the classic movie The Thing where one of the scientists (I believe it was Dr. Blair) looks through a microscope at a sample taken from a mutilated corpse. We don't immediately know what he sees or know what he knows ... but we want to. He has begun to understand the mystery and we want to as well.
This short period of unknowing, between the audience seeing the character's reaction to the new knowledge and finding out what it is, creates tension. I wanted to take Dr. Blair by the shoulders, shake him, and say something suitably melodramatic like, "What is it?! Tell me!"
3. (No dramatic irony) The characters and the audience/reader have the same amount of information.
A scene can be tense even though it lacks dramatic irony. In this case the tension will be produced by other factors, factors like a ticking clock, a clear statement of the stakes, and conflict. Lets take a closer look at these.
Preconditions For Suspense
In order for a story/yarn/tale to be suspenseful, the following must be in place:
What is conflict? How is it generated/produced?
Conflict = (a character's goal) + (opposition to that goal)
That is, conflict results from the clash of two things:
(a) What the hero desires or needs. His/her goal.
(b) Something fearful that opposes the hero, something that can prevent him from getting what he wants/needs. 
In order to create suspense, the stakes of the conflict should be clearly spelled out well in advance. That is, before the hero is actually menaced by the danger.
The general stakes for most horror movies are as follows:
The hero wins: the hero (and possibly one or more other characters) escape the evil and live.
The hero loses: the hero fails to escape the evil and they die.
By the way, Joss Whedon and Drew Goddard's movie, The Cabin in the Woods, gives these stakes an ironic twist. It's a huge spoiler, so I'll talk about it in a footnote. Don't look if you haven't seen the movie and want the ending to be a surprise. 
3. A ticking clock.
“Make them laugh, make them cry, make them wait.”
You need to build up pressure/tension which means that, in some way or other, the characters must be racing against a clock. This both sets a deadline and gives the character time to plan, agonize and, finally, fight; time in which the reader can agonize.
Lee Goldberg said:
"[To create suspense one needs] A ticking clock that escalates the conflict either within a person or between two characters, and that's an essential part of suspense, its the kindling that creates suspense, the conflict between two characters and the outside force or the outside pressure that makes that conflict even greater and then--boom!--you have the inevitable scary, frightening, exciting climactic verbal explosion."
An Aside: Another way to create suspense: raise a question
When we talk about creating suspense we (of course) are talking about an emotional state that exists within a reader/viewer/listener. Generally we try to evoke this emotional state by getting our readers to identify with our characters--especially our hero/protagonist. We make it clear what the protagonist wants/needs and then we force the protagonist into danger as he/she tries to attain their goal.
Yes, certainly, this kind of conflict creates suspense. But I would like to point out that there is another way to create suspense: raise a question.
Lee Child is a great proponent of this method. He even goes so far as to say that it doesn't especially matter whether your readers care about the subject matter; there is something about a question being raised that makes us want to know the answer.
And you know what? He's right!
The other day I read a fabulous short story--"In The Cave" by Tessa Hadley--where the suspense was generated by a question the storyteller asked: What happened to break the hero's infatuation with her almost-boyfriend?
Yes, sure, I read on because the writing was enchanting, and because of the conflict generated by the clash of the protagonist's current state of affairs and the state of affairs she desired for herself. But, mostly, I read on because I wanted to know the answer to the question the storyteller had raised in the first paragraph: Why hadn't it worked out between the protagonist and her companion?
That's it! Here's a writing challenge (I'm challenging myself with this as well): Build some suspense in your writing today.
1. Suspense, Wikipedia.
2. Secrets to Writing Top Suspense, Lee Goldberg.
3. In The Cabin in the Woods Joss Whedon and Drew Goddard put an ironic twist on the stakes making it the case that if the hero (Marty) wins and escapes the evil then the world will end. On the other hand, if the hero allows himself to be killed by the evil then the world will be safe ... and five other people will be brutally murdered every single year the world stays that way.
Google hangout: Secrets to writing top suspense:
Guest post on Lee Goldberg's website about suspense. Post is by Libby Hellmann:
Photo credit: "Bern - Switzerland" by *Light Painting* under Creative Commons Attribution 2.0.