In fairy tales characters are straightforward good or evil, so that the audience does not get confused. Shawshank is the sort of prison where convicts are "good" and guards are "evil", so that you know immediately who to root for.

The main characters in the movie are Andy and Red, respectively played by Robbins and Freeman. Robbins is an actor with a natural creepiness that cannot be neutralized even by the most positive role. We are supposed to root for Andy, but the ambiguity about his guilt is kept going two thirds into the movie and does not help to establish him as a sympathetic character. Freeman is by now a stand-in for God. He conveys gravitas and righteousness, even to a murderer. He deserves a happy ending against all odds.

The story unfolds extremely slowly, with the usual bonding and strife among inmates and abusive guards stirring the mix. The only plot twist is an absurd conversation between Andy and the prison's warden. In possession of vital information about his case, allegedly clever Andy chooses to share it with the evil warden, rather than with his lawyer. This is clearly an idiotic decision, but so far looks like nobody noticed.

However, the simple tale of a man convicted for a double murder, taking place almost exclusively within the prison walls is hardly material for a masterpiece. Even more so when based on a Stephen King novel. King may be massively famous, but subtle he is not.

I guess the reasons for the huge popularity of this movie lies exactly in its simple structure and exploitation of basic emotions and human qualities such as endurance, hope, faith and integrity.

The main message of a film is an even more basic "Everything is going to be alright". That is, if you consider decades spent in prison as a metaphor for life.