At the beginning of Fahrenheit 451, Guy Montag is a fireman who loves his job, as we learn from the opening line:
"It was a special pleasure to see things eaten, to see things blackened and changed."
By Part Two of the novel, however, Montag's attitude towards his job has changed significantly because Montag is plotting with Faber, a man whom he once listed in his "future investigations" file. Together, they have developed an "insidious plan" to bring down the fireman system from the inside by planting books in the home of other firemen and then anonymously turning in the alarm. Montag is able to put this plan into action in Part Three of the novel when he frames a fireman called Black.
Another area of change for Montag relates to his sense of happiness. When the novel begins, for instance, Montag does not doubt his happiness. In fact, when Clarisse asks him if he is happy, he dismisses her question as "nonsense." But once this issue has been brought to his attention, he cannot let it go and it prompts an identity crisis:
"He was not happy. He said the words to himself. He recognized this as the true state of affairs."
As this crisis deepens, Montag's quest to find happiness has dramatically changed his life: he has left his profession, hoards books and has realised that his marriage to Mildred is empty and loveless.