Into the Wild differencesk

The movie definitely takes Alex's view point; explaining things through his point of view and agreeing with basically everything he says which would be very hard to do.  The things they have him saying in the movie are not objective, they are completely subjective hinging on Alex's view.  

The  movie does not follow the exact chronological order that the book follows.

The move also ad-libs some things as well, interpreting parts of Alex's journal to fit the plot of the movie

The belt that it shows him punching new holes in the bus was in fact given away before he headed off for Alaska