The novel never mentions him having any hobbies; the opening passage seems to indicate that he is consumed by his job. It is everything to him, so much that a "fiery grin still gripped his face muscles" as he dreamt about burning things at night in his sleep. He ridicules and mocks Mildred's t.v. walls, so we get the impression that he isn't really into those either. His dark secret hobby though is his fascination with books. When he enters his house for the first time we meet him, it alludes to the fact that he already has a book hidden in "the ventilator grille in the hall." So, that is his hobby and interest.
We don't know where he came from; he mentions his childhood only in a memory brought on by Clarisse's face; he recalls a power outage where his mother and he had sat in candlelight "alone, transformed, hoping that the power might not come on again too soon." Of religion, there is no mentioning of one. Given the descriptions of his society, it is easy to infer that it is a pretty godless society; entertainment and busyness is their religion. Of children, he evades the issue when Clarisse asks, stating, "my wife...never wanted any children at all." So, those are some of the things that would give us more information about Montag, but in reality they don't; it is very little information.
As the novel progresses, we learn that he is a thinker, a deeply troubled man, passionate and questioning. He is also very brave, willing to commit sabotage in order to change the world. He is an unlikely hero, who sets out to incorporate changes into his life, but ends up with more than he ever bargained for.