The Book vs The Movie
Aron Ralston was a completely different person in the film.
In the book, Aron seems to be very calm, stable, and controlled. He describes many things analytically and thoroughly. In the movie, he seems far more outgoing and reckless. There is one scene in the film where he crashes on his bike and starts laughing and taking pictures of himself on the ground. This has the effect of making readers feel more sorry for Aron while reading the book.
The women Aron met on the trail were different too.
In the film adaptation, the two women that Aron meets before becoming trapped mention as him if he likes to go to parties and spend a lot of time talking about a party that they are hosting the next day. They seem to spend a lot of time talking about Aron in private while the book never implies anything like this. In the book, the women carry significantly less importance. When Aron becomes trapped, he spends less time thinking about them and more time thinking about his family.
The book does not spend as much time focusing on these women.
There is a scene in the film where the three go climbing through a canyon and swim in an underground lake. In the book, this never happens. When Aron begins to hallucinate as he becomes more dehydrated, he reflects on this moment and thinks of them a lot. This might have been done to attract a larger audience because the book almost exclusively focuses on one man and having a movie be all about a single person would not be very interesting. As a result, the director probably wanted to give more personality to the few people who were a part of Aron's story.
The book felt more graphic and disturbing.
It's hard to express through a purely visual medium the pain that Aron went through when he amputated his own arm in real life. Overall, this scene was about equal in terms of intensity as it was in the book. Aron had to break his own bone to free himself and the scene where he cuts his nerve is just horrifying. However, the film does not effectively show the pain of dehydration. Much of the book is about the suffering of having no water but this is briefly addressed in the movie.
The film never showed Aron's childhood.
In the book, Aron tells stories of his childhood and explains how he became interested in adventuring. He tells a story of one huge turning point in his life when he encounters a bear and believes that he is going to die. He explains how he believes that this taught him to respect nature more. The film never mentions any of this and it makes it harder to feel as attached to Aron when bad things eventually happen to him.