The key to any fiction is tension. Romantic tension, professional tension, survival, etc. In a thriller, the tension is primarily adversarial in nature. Here, whether our protagonist is striving for some sort of reconciliation, kumbaya moment with the antagonist or is willing to stop them at all cost, the thriller is driven forward by the intensity of the conflict between our hero and our villain. The more intense the conflict the better.
Some thrillers open with an inciting moment that amps the conflict up to eleven right from the start, but maintaining that level of intensity through an entire novel can be challenging. You must be sure that your story has enough constant tension to carry it through to the end. This can also sometimes be a little much for readers, who may need to set your book aside if only to catch their own breath for a moment. There is then the slow burn: a build-up of tension from what may seem an innocuous incident at the beginning, mounting through a series of cause and effect events of ever greater intensity that eventually lead to the all-out war of good or bad outcome.
One method of maintaining tension throughout your book is the ticking clock. Whether it is a loved one dying of a rare disease the cure of which must be found or gulp; an actual ticking bomb (or multiple bombs) that must be found before it explodes; or simply a deadline by which the antagonist must complete their preparations in order to meet some window of opportunity for their plan to succeed. The last one places the ticking clock not only on our hero but the villain as well.
In contrast to, say, the mystery, the romance, or the historical, thrillers generally have few quiet, introspective moments. Character development must be done on the fly, in the midst of conflict and tension, the quiet moments brief and still filled with the tension of an oncoming missile which may not be here yet, but whose whistle we can hear bearing down on us from the air.
The greatest challenge, I think, in writing the thriller is finding the right pace of building tension, and maintaining tension throughout the book. This is what the thriller writer must focus on primarily.
Do you have some examples in thrillers you've enjoyed or, most importantly, learned from? Let me know in the comments below.