One of the themes that is expressed through Montag's development as a character is the theme of loneliness. At first, Montag is a passive, thoughtless tool of the government. He has his firemen pals and his wife. He is not lonely. He doesn't question things and is blissfully ignorant as a result. But, there has always been a hint of curiosity about books (recall his first meeting with Faber a year prior to his conversations with Clarisse). So, he has always had a faint inclination to question things. After speaking with Clarisse, this inclination is awakened and his curiosity grows exponentially. Becoming a free thinker, Montag is alienated from everyone except Faber. Montag is alienated from his wife because she does not want things to change. He is alienated from his entire society because that society is willing to be complacent and remain thoughtless in their passive lives. In becoming a free thinker, he also becomes lonely.
As Montag's curiosity increases, his boldness increases as well. He is audacious enough to read poetry to Millie's guests. He is bold enough to steal more literature. In the end, he is brave enough to take down Beatty, the symbol of authority and oppression.
This ties into another theme which is that of individual freedom: physically, politically, and psychologically. Montag's awakening from thoughtless tool to critical individual represents this thematic transition to becoming a critical thinker, one who is no longer brainwashed by the state (government).
In the end, not only does Montag embrace the power of knowledge and literature, he becomes literature. He also finds others like him and this ends his loneliness. In becoming literature (a book), this underscores the idea/theme that literature is "alive" in the sense that the knowledge and ideas therein can inspire a person and/or people to change, grow, and develop into something more.