Denouement...
Doubtless this has been brought up before ... I think many of the changes in the film version work well: e.g. Andy's ongoing conspiracy with a single warden; his ability to squirrel cash away seems more authentic than just relying on accounts made long ago by a friend who has since passed, etc.
In the novella (& it stands to reason that a post entitled "denouement" will contain spoilers) Red is finally paroled knowing only about the rock Andy's friend used to hide the safety deposit key & its approximate location. Andy never suggests he follow, seek out the rock, or join him in Mexico.
As Red begins to consider committing a minor crime in order to return to prison, he starts seeking out the rock instead. Not for any particular reason, except perhaps to verify that Andy really got away. When he finds it, Andy's note, invitation & traveling money are a complete surprise.
And so much more satisfying, I thought. After all, Red left prison still in prison, as it were. Andy's escape & thoughtfulness liberated not just himself, but Red, too.
When Andy tells Red to look him up well before his escape it reduces all of the tension & surprise.
Of course that's just me. I liked King's ending to The Mist much better than the film's.