A thriller is what the name suggests: thrilling. Thriller novels are characterized by a fast pace, tension, excitement and the anticipation of what comes next. In a psychological thriller, the tension is cerebral. The excitement is based on the mental process of the characters and, consequently, the reader. Psychological thrillers are like fast-paced mysteries but much more complex, and the villain uses mental as well as physical manipulation.
Engage the Reader -- Quickly
Begin the story with a bang. Some novels have prologues that set the stage and provide a backstory. Even if you don't include it in your final manuscript, consider sketching a prologue, if only to help you develop the backstory and set the stage. To set the tone for your thriller, put the reader in the middle of the action and create tension as soon as possible. If you are writing from viewpoints of multiple characters, introduce different perspectives and any subplots with a clear vantage point within the action. Create an immediate and intense sense of urgency using action verbs and vivid descriptions not only of what is going on, but of who is involved and surroundings that may contribute to suspense.
Develop Your Characters
The reader will not be thrilled by the action if he or she does not care about the characters and what happens to them. If the hero is in trouble and the villain is closing in, readers will not feel the suspense if they do not root for the hero and are not repulsed by the villain. Your villain should use mental tactics that defy convention, and your hero should be extraordinary in some capacity. Create emotional connections with characters, even minimal characters, by providing physical details and some sort of backstory. For example, readers will be more affected if you tell them that young newlyweds on their honeymoon were hit by a speeding car than if you just say a car hit two people.
Leave The Reader Hanging
Thrillers are characterized by cliff-hangers. Connect scene to scene, and don't let the action drop. Always set up what's coming next with suspense. End each chapter with a glimpse into the next, and create tension at the last minute that will keep the reader turning pages. In a psychological thriller, the action will be primarily mental and emotional. Psychological manipulation and mind tricks will keep the reader guessing. For example, a cliff-hanger might be suspenseful as a character makes a discovery, such as your hero finding that he's been lied to or deceived.
Timing Is Everything
Time itself is a character in a thriller. Use time to your advantage, and engage the reader by using time as a catalyst for the action. For example, your hero must race to save the love interest before the villain succeeds. Just as comedy is all about timing, suspenseful moments also must be timed perfectly. If your villain is playing a game of cat-and-mouse with your hero, carefully plan the timing of each moment the villain strikes. Don't let the reader get too comfortable. Plan light moments of relief only to plunge the reader back into fast-paced action.