Two good examples of themes expressed through characterization are Mildred, Montag's wife, and Clarisse, the neighbor girl he befriends.
Mildred stands for the theme of collectivism through conditioning. She has no understanding of ideas, instead only concerning herself with the trivial minutia of television programming and keeping up with current trends. Instead of cultivating a life, she fixates on shallow entertainment, only seeing other humans in person when they all share a television experience. By immersing herself in social norms, Mildred trades personal freedom for the superficial comfort of controlled thought and action.
Clarisse stands for the theme of individualism. She doesn't attend the indoctrinating school, instead taking long walks and observing people as they interact. Her ideas are more concerned with the way she sees the world, focused almost entirely on her own personal, subjective experience. By ignoring social norms and creating a system of values based on her own thoughts and experiences, Clarisse rejects the collective mentality of society and embraces personal freedom.