In non-fictional form, many may find it difficult to just let it all out. Imagine sitting down with a stranger--or even a friend--and having them dish out their deepest fears and regrets. You'd eventually think less about how you can relate, and more about how this person's got some serious problems. When we sit down to share our real-life stories--as we've done in our personal essays--honesty is something we might dance around or only brush up against when convenient.
When considering fictional writing, honesty can take any form. It can be an antagonist who bears striking resemblance to your horrific 4th grade teacher who was terrible and awful and is the direct cause of any educational problem you've had since. For Douglas Thayer, it appears to be taking the form of inner monologue. Kyle waxes on about his thoughts, flowing in and out of memories and ideas that have clearly been hand-picked by Thayer for different purposes.
Stream of consciousness writing, as found in The Catcher in the Rye and Ulysses, is an easy and effective way to be honest with your audience, or at least let your character be honest with them. I don't imagine Kyle is Douglas Thayer and that he's just barfing out his own life story through another character, so he's most likely using Kyle to be honest about his own personal thoughts and about what he thinks teenage boys think about. Doing this in the non-fiction form would be very difficult.
By avoiding the nasty trap of how much honesty is too much honesty, writers can use fiction to breach any number of topics with greater efficiency.