Effective terrorists manage to keep their victims in constant uncertainty. Will the next attack be a car bomb at the mall, sarin gas in the subway, or anthrax in the mail? Their victims live in constant tension, knowing that they could be hit at any moment from any direction. Suspense writers need to be able to do the same thing to our victims, er, readers.
This is the hardest rule to implement, but also the most necessary. In one sense, it’s impossible: Before readers even pick up your book, they know that your hero(es) are almost certain to prevail in the end. If terrorists are plotting to assassinate the president, they’re very likely to fail. If a serial murderer is stalking a quiet college campus, the main character’s daughter probably will be safe in the end.
The trick is to keep surprising readers along the way. For example, in The Day of the Jackal Frederick Forsyth tells the story of a fictional attempt to kill Charles de Gaulle. Since de Gaulle was not actually assassinated, we know right off the bat how the story will end. But we don’t know why it will end that way. Forsyth does a magnificent job of keeping us off-balance throughout. He creates twist after twist as the brilliant assassin, the Jackal, and French intelligence agents play a lethal game of hide-and-seek. Even though we know who will win, we’re enthralled by the game.
There are plenty of good ways to create plot twists. Forsyth does it by making both the Jackal and his pursuers smarter than we are; we can’t see their actions coming, but they make perfect sense in retrospect. Unfortunately, that only works if you’re as smart as Forsyth. An easier technique is to throw in a critical fact known to the characters (or some of them anyway), but not the reader. For instance, your FBI agent protagonist could inexplicably destroy evidence that would put a mob boss away for life. Why? Because the agent and the mobster—but not the reader—know that the agent’s illegitimate son works for a mob-owned business and would be killed if the mobster went to jail.
You can also put your characters into unpredictable situations and see how they respond: What happens when your blue-collar American detective has to find a killer who disappeared into the high reaches Brazil’s aristocracy? Does he take a crash course in Portuguese and get himself a menial job at the estate of one of the killer’s relatives? Does he find a Brazilian cop who knows the dirty laundry of his country’s privileged class and would like nothing better than to see one of them brought to justice? Or maybe instead of a cop, the detective finds a well-connected drug dealer with whom he makes a Faustian bargain?
One caution: Not all plot twists are your friends. Two in particular should be used sparingly if at all. The first is the Character Lobotomy, in which the author dramatically drops a character’s IQ in order to make the character do something inexplicably stupid to move the plot along. The most common (but by no means only) example is the Talking Killer scene found near the end of most James Bond movies and Harry Potter books. The villain has the hero trapped and need only kill him to be certain of victory, but the villain doesn't do it! Instead, he says in effect, “Shut up, you worm, and let me explain the plot to you before your improbable escape!”
The second treacherous plot twist is the Convenient Coincidence: the scene where the heroine arrives in the split second between the end of the Talking Killer’s speech and the moment he murders the hero, for example. Or the key inscription that can be read only because a character happens to be fluent in Medieval Urdu. Or the Bible in the protagonist’s vest pocket that stops what by all rights should be a fatal bullet. Again, it’s best to avoid these.
The problem with both Character Lobotomies and Convenient Coincidences, of course, is that they make your story less believable. The minute readers stop believing your story, you’ve lost them. The spell is broken. They sit back from the edges of their seats, roll their eyes, chuckle, and toss your book onto a coffee table or into the dreaded library donation box. Not a pretty mental picture, is it? Don’t let it happen to you.