Fear is a very powerful influence in Fahrenheit 451 and we see numerous examples of this throughout the novel. Fear of the Mechanical Hound has a profound effect on Montag, for example. When the Hound growls at him, Montag's reaction betrays his impending sense of rebellion to Captain Beatty, as we see from their conversation:
"I wouldn't want to be its next victim."
"Why? You got a guilty conscience about something?"
Similarly, in Part Three of the novel, fear of being caught and killed by the Hound drives Montag to flee the city. This not only saves his life (because the city is later destroyed) but also puts him into the path of Granger and the other professors. These men are significant because they have memorized books in an attempt to preserve knowledge for future generations.
Finally, fear is also a driving force in Mildred's life. Her fear of Montag's book collection, for example, drives Mildred to report her husband to the authorities. She also leaves the house without even saying goodbye to Montag. In her imagined death (which occurs as a result of these events), she dies alone in a hotel room, a few seconds after realizing the emptiness and isolation of her existence.