In Fahrenheit 451, Ray Bradbury uses characterization and contrast to create the theme of a lack of identity.
Montag is a character who lacks an individual identity, at least at the beginning of the novel. He is a fireman, a man who actually starts fires to burn banned books (and essentially all books are banned). His life is just like the lives of every other fireman. They look alike. They act alike. Montag is just another cog in the wheel. He asks no questions, thinks no unique thoughts. He just does what he is told.
But then, Montag meets Clarisse, who definitely has an identity of her own. The contrast between them is startling. Clarisse observes the world around her. She thinks. She cares. She wonders. She enjoys. She is her own person, and she dares to be different. Montag doesn't quite know how to handle her, yet she teaches him what it means to be an individual, what it means to stand out from the crowd.
Clarisse also contrasts with Montag's wife, Mildred, who lacks a particular identity. Mildred's life is empty and meaningless. She spends much of her time distracting herself with "parlor walls," or video screens, which provide continual programming. Even though society considers Mildred a "model citizen," she refuses to think for herself and is clearly unhappy.
Toward the beginning of the novel, Mildred attempts suicide by swallowing an excess of sleeping pills. The technicians who save her are nonchalant about the whole thing. They see nine or ten such cases a night, and they do the same thing for every one. To them, Mildred is not an individual, but just another stomach to be pumped.
By the end of the novel, we notice a sharp contrast in Montag's developed character. He has learned about books and knowledge and individuality. He turns his back on being a fireman and on society as a whole. He has become an individual in his own right.