Shortlist.com recently published 20 Things You Probably Didn't Know About The Shawshank Redemption. Here is my summary of them in honor of the film's 20th anniversary:

The film is based on Stephen King's novella, Rita Hayworth and the Shawshank Redemption which is contained in his book, Different Seasons. The character, Red, played by Morgan Freeman, was a red-headed, white Irishman in the novella. Stephen King received $5000 for the movie rights, but never cashed the check. The American Humane Society did not allow the crew to kill the maggots that Brook's pet crow, Jake, ate during the film. The crew had to find maggots that died from natural causes. The role of Tommy Williams, played by Gill Bellows, was meant for Brad Pitt, who opted for a role in Interview with the Vampire during the same year. Stephen King's trademark number 237 (The Shining's room 237, Stand By Me collection of $2.37, e.g.) is in The Shawshank Redemption. Red's cell number is 237. The film was on US television channels for 151 hours of airtime during 2013. An amazing amount of time for a 20 year old movie. Although set in Maine, the movie was filmed in Mansfield, Ashland and Upper Sandusky, Ohio. These three towns continue to thrive on tourist business due to the film. Tim Robbins initially refused to immerse himself in the muddy water after he escaped from the tunnel. A chemist had previously tested the water as “lethal.” Robbins decided to do the scene anyway in the end. The escape tunnel was made from a mix of sawdust, chocolate syrup and water. Red's young mugshot pictures were actually pictures of Freeman's son Alfonso. Andy and Red's opening scene where they chat in the prison yard took nine hours to film. Since Red was required to toss a ball throughout this scene, his arm became so sore that he had to wear a sling the next day. Andy's rock pick was hidden in his carved-out bible at the Book of Exodus, which details the escape of the Jews from Egypt. In Finland, the film was called Rita Hayworth – The Key to Escape, in Israel it was called Walls of Hope, in Taiwan it was 1995: Fantastic, and in Italy, The Wings of Freedom. Other actors considered for the part of Andy Dufresne: Tom Hanks (took Forrest Gump instead), Kevin Costner (did Waterworld), Tom Cruise, Nicholas Cage, Johhny Depp, and Charlie Sheen. Only two women have speaking roles in the film. The director dropped “Rita Hayworth” from the film's title because he felt people would think it was a Rita Hayworth biopic. According to the novella, Brooks was in prison for murdering his wife and daughter after a losing streak at poker. Red's life term was for murdering his wife by disabling her brakes, which accidentally kills a neighbor and child along with her. In the novella, Andy's cell is adourned with posters of Jayne Mansfield, Linda Ronstadt, Hazel Court, and Rita Hayworth. In the film, the posters are of Rita Hayworth, Marilyn Monroe, and Raquel Welch.