Quite clearly, the best place to start when comparing the film version of the novel and the novel itself is to identify and analyse the various differences that you can identify. It is very important to ask yourself why these differences occur, and what they add and take away from the translation of the novel to the screen. Consider the following differences just to give you some ideas:
1) The naming of characters is particularly significant in this novel. However, Michael's baby sister is named Joy in the book, but Grace in the movie. Which name is better do you think? What do the names suggest, and how do they relate to the themes of the novel as a whole, especially the idea that people can be healed by love?
2) Linked to the first point, Grace is the name given to Michael's baby sister in the movie. This name comes from an old woman who told Michael about treating arthritis. The character of Grace is developed in the film, and she and Michael have a strong friendship. However, she dies before Michael visits her in the hospital. What does this relationship (that doesn't occur in the book) add or take away from the film as a whole and the various themes that it contains?
3) Finally you might like to consider the way that the poetry of Blake and allusions to his work are central to this novel. Is there a difference in the way such allusions are presented in the novel and the film? Which one works better in terms of how these allusions are made and established?
I hope that these three pointers give you some ideas to start thinking about analysing the differences between the novel and film version of this excellent story. Good luck!