The first difference in the movie, is the scene where older prisoners are betting on which one of the new inmates will snap first, This scene was never in mentioned in the book. This scene may have been added to give the opening a bit of comic relief and a chance to give Morgan Freeman a break in his narration. This also shows the audience that Andy has it easier than other inmates but life in prison is never easy. Also this gives the viewers a chance to sympathize with prison life, while also making third person. This constant change of narrative to third person gives the movie a nice change of pass. Brooks was one character that differed from the short story. In both the movie and short story he is an old man in prison who has a pet bird that he took care of named Jake. However the story didn't go into much detail what happened to him in his life after prison, but in the story Jake his bird ended up dying after he set it free because it starved to death. Although in the story Brook's adjustment period is not told, however in the movie this struggle is shown. It actually shows him hanging himself after craving Brooks was here in the wood.
Another change is to the character Tommy Williams. In the story Warden Hadley had Tommy transferred to a different prison while Andy was in solitary confinement so that he wouldn't be able to testify on his behave. However in the movie Warden Hadley kills with a corrupt prison guard.
Another example of differences evident in the book to the movie is the character Byron Hadley. This unpleasant individual has two different fates, in the movie Byron is arrested. Although in the story, he dies of a heart attack probably the result of the shock that he might be an inmate at Shawshank. The movie had a lighter feel to it this reason the did not have him dying in the movie was to avoid the sadness of the viewers, that death brings. It is easier to just "arrest" a person then to kill there character off. Not killing him, allows them to have the flexable to bring him back for another movie, which we know was not made.
Lastly the escape-ending in the story is the same but some details are explained. Since Red is the narrator in the short story, he could not be sure about some of the details of Andy's escape. For example the change of his prison clothes to regular clothes was not explained in the short story. In the movie Andy is shown with a plastic bag filled with Wardens stolen clothes and bank information tied to ankle by rope as he crawled through the miles of sewer. Shawshank ends in uncertainty, where the reader is left to wonder if Red finds Andy. On the other hand the movie ends with Red walking on the beach in the distance towards Andy.