Symbolism, Imagery, Allegory
The complete name of Steve's screenplay is Monster: The Story of My Miserable Life. He's on trial for murder, so the miserable part is pretty self-explanatory, but if you're unsure about monster, check out our discussion of it elsewhere in this "Symbols" section. We'll wait.
Okay, you got it? Onward.
Steve's screenplay is more than just an original way to write a young adult novel (but props to Walter Dean Myers anyway)—Steve actually needs the screenplay. At one point he writes, "I need it more and more. The movie is more real in so many ways than the life I am leading. No, that's not true. I just desperately wish this was only a movie" (15.14).
Reread the last sentence in that passage. It exposes the relationship of Steve's screenplay to Steve's actual life. His movie takes Steve away from his inner WWF championship, it allows him to look at things from an outside perspective so he can feel like one of the classroom kids watching the show, rather than the prisoner hunched in his chair. It even allows him to "[depict] his neighborhood and environment in a positive manner," like he does in film class (18.176).
Not everything is positive in the screenplay of course, but that's not Steve's goal anyway. His goal is to remain sane throughout his ordeal, and looking at things objectively helps him do that by helping him recognize his own humanity within the chaos of court and such. In his movie, he can show shots of the trial and glimpses into his past—and this helps him connect with himself and his life beyond prison.
The screenplay keeps Steve's volcano under control. It helps Steve "make sense of [his life]," because "to get used to this I will have to give up what I think is real and take up something else" (1.7). Too bad some things never seem to make complete sense, whether he includes them in the screenplay or not.