The movie The Reluctant Fundamentalist is based on the novel by Mohsin Hamid, but it is really quite different in characterization and even in its plot. Let's take a look at some of the primary differences.
In the novel, the protagonist, Changez, narrates in the first person. This is not feasible in the movie, so we see Changez more from the outside instead of hearing his perspective directly. Changez is a more ambiguous character in the book than in the movie as well.
The movie also shows a different version of Changez's love interest, Erica. She is a visual artist instead of a novelist, and in the book, she has deep psychological issues that do not appear as strongly in the movie. The book suggests that she commits suicide, but in the movie, she and Changez merely split over an argument about a piece of art.
The movie adds a great deal of detail to the unnamed American we see in the novel. In the novel, Changez talks to the man in a cafe and explains his time in the U.S. In the movie, this American has a name and a back story all his own and plays a much greater role in the plot as a secret agent out to find a kidnapped professor.
Finally, the movie shows a great deal more violence and prejudice than is described in the novel. In the novel, for instance, we hear of Changez's difficulties after the September 11th attacks, but in the movie, these are dramatized much more vividly.