Most of the characters in Fahrenheit 451 have completely subverted their true natures to the forces that society has put upon them. In their society, the controlling forces numb everyones' minds through entertainment, and sticking them full of useless facts. As a result, they never think, and all behave and act the same, like they are one person. And because of this, they have a very unhealthy society that is fixated on matieralism and war. Once you remove values, and personal integrity from a society, all that is left is warring for power, and we see that happening in their society.
Montag has stirrings of individualism; he feels discontented with society. And, every time he tries to deny those feelings of frustration, he just ends up unhealthy and angry. For example, when he burns down Mrs. Blake's house, he is super upset about it, but tries to quench his feelings and just does the job anyway. Later, his true self comes out, and he feels so guilty that he can't even go into work the next day. It isn't until Montag embraces his feelings of individuality that he feel truly free. As he listens to his inner voice, he is happy. He loses everything around him, but because he has listened to himself, he still has his conscience and individuality in tact, and it doesn't matter as much.
Faber too tried to stifle his inner voice, for years. When he starts making plans with Montag, he is elated, free, and feels relief and happiness for the first time in years. The only character we see that didn't fit society's molds was Clarisse. She constantly listened to herself, and was in tune with her personality, and she was completely happy.
I hope that those thoughts help to get you started; good luck!