Surprise and Suspense
Alfred Hitchcock gave a famous definition of the difference between surprise and suspense. It boils down to this: If a bunch of guys are playing poker and suddenly a bomb goes off under the table, that's a surprise. It's not what the viewer expects. If, however, the viewer knows the bomb is there from the start, and watches the timer ticking down toward zero while the men play on, oblivious, that's suspense.
The definition assumes a couple of things, not least that the viewer (or reader) actually cares about what happens when the bomb goes off. Note that it doesn't actually matter much whether the reader cares because the hero is playing poker and the reader doesn't want him blown up, or because the villain is supposed to be in the game but hasn't arrived yet and the reader/viewer is hoping that the bomb won't go off until he gets there.
It's a great definition, and it illustrates one of the basic techniques for creating tension or suspense: let the reader know more than any one character knows, so that the reader can see trouble coming a long way off. But it's not quite as simple as that, and trying to apply this technique without some level of understanding often results in false suspense.
For instance, take a slightly different situation: the heroine has discovered a plan to kidnap her son; she calls his cell phone, but there's no answer. The kid frequently forgets to charge the phone, though, so he might still be fine. She jumps in the car and tears across town to his last known location -
- and halfway there, she gets stopped for ten minutes by one of those hundred-car freight trains going by.
That's false suspense. The train doesn't just stop the heroine; it stops the story, because the story doesn't progress until the heroine gets where she's going and a) finds her son, b) doesn't find her son, but finds a clue as to where he's gone, or c) arrives just in time to foil (or not foil) the kidnap attempt. Yes, waiting for the train makes it more likely that she won't get there in time, but dragging out the trip for no story-related reason annoys most readers. So you don't want to do that.
The basic elements of suspense are the same as for any story: a protagonist we care about and something important at stake. What creates the suspense is the reader's awareness of some reason why the protagonist is very likely to fail. It can be something the protagonist doesn't know about, like the bomb under the table, or it can be something the protagonist does know about, like his own fear of heights or alcoholism. One can get a tremendous amount of tension and suspense out of a scene in which a former alcoholic, pushed almost to the edge, hesitates in front of the door to a bar, or studies the cocktail tray at a big party.
Usually, a suspenseful scene has some sort of time constraint - the bomb under the poker table wouldn't be very suspenseful if it was just sitting there, unprimed, with no timer. It doesn't have to be a short, specific time constraint, either; "…before the plane runs out of fuel" or "…before the virus mutates into its deadly form" work just as well as "…before the bomb goes off at 12:23 p.m.
But time constraints aren't always necessary; the recovering alcoholic who is resisting that moment of temptation doesn't have any particular deadline. The lack of deadline is, in fact, part of the point - resisting temptation is something that he's going to be facing for the rest of his life.
One can also create tension by limiting the amount of information the reader and/or protagonist has, doling out important details with agonizing slowness. The trouble with this technique is that it is very easy to limit the information too much, and end up with mere surprise, rather than suspense. In other words, if you're going to create suspense by limiting what you tell the reader and only revealing it slowly, the reader needs to know that there are important things you're not telling him/her. You also have to get the timing right; if the revelations come along at too slow a pace, eventually the reader is likely to give up.
One thing you absolutely do not want to do (except possibly in a totally over-the-top parody piece, and even then I'd advise caution) is a deliberate false-tension fake-out - the sort where the protagonist screams, blood spurts, and after two pages of backstory (his life flashing before his eyes?) the writer reveals that the protagonist has just cut himself shaving. This kind of thing destroys the reader's trust in the author (apart from obvious parody), and generally leads to instant wall-flinging.
