The movie ‘The Shawshank Redemption’ stays true to the short story by Stephen King. In fact it is a matter of great surprise how much the story has to offer as is that makes it into Frank Darabont’s screenplay. Yet, Frank Darabont manages to leave his mark with some nice changes to the overall scheme of things. (Spoilers ahead)

1. Character of Tommy: In the book, Tommy takes a deal from the warden and moves to a better prison, however in the movie, he is killed by warden. The change in the movie is much better as it provides a drama (the manner in which he his killed) and adds another feather to the cap of ruthlessness of the warden (no prisoners approach). 
2. Getaway plan: In the book, the plan is a bit complicated with Andy having an associate outside and the details to his getaway plan (once he escapes the prison) is hidden under the black volcanic rock in the field of Buxton. While the aspect of both Andy and Red, coming to the same field to look for their final escape is interesting, the movie simplifies the plan, with Andy taking charge of everything (which is actually true to his character, not depending on anyone),but still leaves instructions to Red, incase he gets out. Most importantly letter to Red is taken as is from the book, as is.
3. Boggs: Andy’s victory over Boggs and sisters is much clear and presented it better than book.
4. Warden and main guard: In the book, this combination changes over time, but in the movie they remain same. This is actually good, to keep the villains same throughout.
5. Time-period: The story drags on for some more years, while the movie understandably reduces it, with the period ending with Lovely Rachael. May be, point 4 is the reason.
6. Overall narration: The story goes back and forth, with Red narrating it non-linearly, rather jumpy. It is justified, as Red with little education and no special skills in writing, can manage it that way. Frank Darabont, should be credited for making this easy on the viewers, with his screenplay nicely picking the threads and tying up into a whole fabric. He does this with élan, without leaving the important and best parts of the book.
7. Warden’s investigation into Andy’s escape: The book scores a few brownie points here with details about a skinny guard going through the Andy’s tunnel and finding out the sewer. In the movie, Red’s narration and Andy’s visuals takes care of it, but the skinny guard (may be from the ‘I gave up drinking’ episode) could have made it effective. Frank might have chosen to opt out, because of film’s duration?
8. Redemption and Revenge: The movie ties this theme neatly, better than the book. Andy’s revenge plot on the Warden is a new addition and provides the last dimension to his Redemption. 
9. Mozart episode: It is not there in the book. Shows the genius of Frank, and builds on ‘Hope is a good thing’ theme.
10. I gave up drinking, Andy getting a mouthorgan for Red, Brooke’s episode: All new, added by Frank.