The Middel

I think one of the biggest changes were in Sam's mind happenend. He "grows up" in the book and is in charge for his deeds. He realized that it is also his baby.
I would ask Sam why he is so stupid to fuck without a condom but fi he didn't do that, this book woulden't exist.
Nick Hornby changed the situation that Sam sees a new hope in his future because of that he have to complete tasks. (He soon will become a father)