In all, there are no wasted characters in this film. Everybody has some connection with everyone else here, even in the slightest involvement in a crowd of jailed inmates egging on the nocturnal whimperings of newly-jailed convicts to the lunch table dialogues and field hand work. The exchanged looks, downcast eyes, or brutal and grim determination of the guards all make this film seem real.
For instance, in Brooks the Librarian, each convict sees their potential fate. A man has been put under lock and key for so long that he's lost all touch with the world outside... or the world outside has lost all need for him. He only has meaning within the prison, and as a final punishment he's stripped of that respect and meaning and tossed out as a used-up old husk into the world.
Why this movie is so powerful
Besides the wonderful acting, rich characters, and powerful dialogue, this movie's got some of the most impressive sets and scenery I've seen for a prison movie. The lighting has just the right shadows and shafts of light, the cell blocks are grungy and grimy and oppressive to the right degree, and the people have taken on a cast like the walls and rocky fields that contain them.
This movie may be centered around prison conditions in the forties, fifties, and sixties, but the lesson still holds true for today's cells and wards - how is it that a prison is supposed to reform what is considered a dangerous criminal? What does it mean to be reformed? Is it enough just to put certain kinds of people away and just wait for them to grind each other up to the point where they lose all hope and all spirit... is it the hope and spirit in these people that is the part that is considered dangerous?
Here's the big question the movie asks: Is there such a thing as a human spirit? From Hadley's perspective, the human spirit is defined by that thing he sets out to break in each and every prisoner under his charge. By Andy Dufresne's definition, it's that thing that keeps him going to matter how long it takes him to get himself free and give every one of his tormentors their comeuppance. It is that thing which cannot be broken.