Suspense is a vital component of writing a crime novel. At its most basic, suspense is based on fear. The reader’s fear. The more anxiety you can make your reader feel, the more successful your story will be.
Conflict. The most important aspect of engaging readers to the extent of them being fearful, is your protagonist and your antagonist. Unless readers not only like your main character, but are mentally cheering them on, why would they be in the least bit bothered when danger threatens and there is the danger the antagonist might win? But then, you can also make the antagonist engaging, too. A likeable rogue with a hard edge that may make the reader overlook him/her. More conflict within the reader.
Strong characters, with flaws that they are fighting to overcome in order to obtain their objective will make your story unputdownable. You must make sure your main character and your antagonist are determined. The one to obtain the objective and the other to make sure that doesn’t happen. Sometimes your protagonist and antagonist will want the same thing. That ups the ante on the conflict. It’s like two people fighting for the love of a third person, except that in a crime novel, it is much nastier and devious.
Time. If you make your characters fight against the clock, the suspense level spikes immediately. If John cannot get to Isabel by 4pm, chances are the killer will get there first and she is next on his list. Can John reach Isabel in time? Make your time short.
There’s a song in Singing in the Rain called Make ’em Laugh. In a crime novel, you make them wait. And you do this by scene cutting; closing the scene on a cliffhanger. You have set up the action but right at the point where the reader is desperate to know what happens, you end the scene and go onto another character doing something different. However, part of the agreement between writer and readers is that the writer does not leave readers hanging for too long. Otherwise they will lose interest and shut the book. Tom Clancy did this very well. He had multiple characters, all facing huge problems and switch from one to the next and so on. If your readers are fully engaged with the characters, the suspense will be incredible. Use time to your advantage.
Foreshadowing. In other words, hints. Sometimes subtle, sometimes up-front. These are a joy to play with because you are trying to lead the reader up the garden path of misleading facts that don’t mean anything. Some hints will, of course, be crucial to the central action of the book. A hint might be, for example, the way a character behaves; something that indicates a state of mind or mindset. Who can forget Bogart in The Caine Mutiny constantly playing with small steel balls? It doesn’t foreshadow an event but sets up the perception of the character’s behaviour and makes what happens later logical.
Foreshadowing for events is less subtle. Perhaps one character is afraid of the dark but is abducted and shut in a cellar with no light. How will he/she deal with the panic on that fear on top of being abducted? Foreshadowing is necessary for suspense and, if done with finesse, can have your readers going in any direction you want to send them.
High Stakes. This is a good accompaniment to the time element above. If the stakes are high and the time is tight, readers are going to be glued to your story with their hearts thumping. Let’s go back to John trying to reach Isabel before the killer does. We already know he must get to her before 4pm. So you introduce a really high stake twist. The hospital is 20 minutes away: he should be ok. But then, he is caught up in a huge accident and its resultant traffic jam halfway to the hospital . Up the stakes, up the suspense.
Plot Twists. (Spoiler Alert) An essential part of a crime novel. Many how-to books advise trying a twist on a twist. The extra one you don’t see coming even if you think you’ve worked out who the killer is. Look at the end of the film Sixth Sense. How many people—including me— didn’t work out that the main character was, in fact a ghost?
How about your protagonist trying to help his brother who has been accused of a murder he didn’t commit. You follow your protagonist’s actions using short time constraints and the high stakes of brotherly love, trying to find evidence to prove the brother innocent and you show the brother twisting painfully on this hook, with his distraught wife and children suffering too. And then the protagonist does find enough evidence to clear his brother. He is declared innocent. The next day, the brother confesses he was guilty. The twist on the twist can also be tightened by how your protagonist finds out the truth. And what they do about it.
Red Herrings and Clues. You need to seed these so carefully. Experienced readers will spot them a mile off. But sometimes, you want them to. And some of the subtle ones will be misleading and a couple of the not-so-subtle ones will be true. I try to plant a significant clue, in passing, early on in the book before the reader has settled down. Throughout, you must be fair to the reader. No sudden appearance of someone at the end who wasn't there at the beginning. The clues must be hidden in plain sight. Drip-feed your clues, don’t give them all at once. And don't answer them all at once, either. Make the reader work for the solution.