Brian's evolution is an element of surprise in Hatchet.
As the novel begins, Brian is a withdrawn, emotionally challenged teenager. He sees his parents' divorce as the worst thing that could happen to him. When Brian boards the plane to Canada, he keeps to himself as he dwells on what happened with his parents. From the moment the pilot suffers his heart attack, Brian must deal with the reality of survival.
I think Brian's change is surprising because it shows how we can adapt when circumstances demand it. When Brian struggles with the moose, withstands the tornado, and plucks out the porcupine's needles, he has no time to dwell on the past. In this setting, being emotionally withdrawn decreases his chance of surviving. Brian's change is surprising because of the amount of strength he displays. He is able to use moments from his life to define his response to the challenges that confront him. For example, he remembers what Perpich, his English teacher, told the class about positive thinking and the ability to overcome overwhelming odds.
It is surprising to see Brian change from a typical teenager into a young man who embraces "tough hope." Brian changes from someone who knew very little about how to function in nature to being knowledgeable about food sources and shelter, as well as how to create weapons for food. Seeing the arc of this change throughout the narrative is surprising.