Montag is, initially, a normal citizen of the government-controlled society. He takes pride in his job, even though it is revealed that he has been stealing books for years.
Later, going to sleep, he would feel the fiery smile still gripped by his face muscles, in the dark.
This shows Montag's joy and his pride, but also the power that society has over him; he cannot stop taking pride in his work, even though it is destructive.
When Clarisse meets him, one of her first judgements is that:
"But you're just a man, after all..."
Despite the power he wields, Montag is as human and as fallible as anyone else. Clarisse recognizes that, but it takes Montag longer; just before he discovers his wife overdosed, his first epiphany occurs:
He felt his smile slide away, melt, fold over, and down on itself like a tallow skin...
This is the first crack in his state-controlled psyche, his state-approved actions, reactions, and opinions.
Many of the adjectives that describe him later focus on his deteriorating mental state, his frantic search for the truth, and his near-hysteria as he realizes how shallow society truly is. He thoughts are "insane," his breathing is "feverish," his voice is "stumbling," and then "firm."
Finally, Montag finds his first true peace as he floats down the river:
The river was very real; it held him comfortably and gave him the time at last, the leisure, to consider...
(Bradbury, Fahrenheit 451, Google Books)
Throughout, the focus is less on Montag the physical man and more on Montag the emotional individual. His description is of less importance than the mental growth he undergoes; each person who sees Montag sees him according to their preconceived notions, until the Montag who watches his city destroyed by bombs is barely a shadow of the book-burner who opened the story.