Symbolism, Imagery, Allegory
The emotional center of Steve's brain is a massive volcano, threatening to erupt and melt his soul, if not his organs… and nobody wants to see that, least of all Steve. So where does he turn? Not to his screenplay (it would mess up the logic and humanity he's so carefully piecing together there—be sure to check out our discussion on it as a symbol elsewhere in this section)… nope, he turns to his journal.
Steve's journal is his place to fall apart, to detail the fear, anxiety, and confusion he feels living in jail and going through his trial. In a situation where he has to remain pretty pulled together, his journal lets him be his whole self. Perhaps this is why the book opens with Steve's journal:
The best time to cry is at night, when the lights are out and someone is being beaten up and screaming for help. That way even if you sniffle a little they won't hear you. If anybody knows that you are crying, they'll start talking about it and soon it'll be your turn to get beat up when the lights go out. (1.1)
Not only can we see Steve's vulnerability in this quote—clearly crying is on his mind—but we can also see how fearful he is of those around him. This shows us that in addition to being physically trapped inside a prison cell, Steve is also emotionally trapped. His journal is the only place he can safely be himself.