According to Billy Bob Thornton, he invented Karl's unique facial expressions and speech patterns, plus ad-libbed the entire "sling blade" speech at the beginning of the film, while looking at himself in a make-up room mirror, waiting to film his scene as a train conductor in The Man Who Broke 1,000 Chains (1987). His scene was later cut from that film.
When Doyle tells Linda that "retards" make him sick, he adds that the same is true for antique furniture and midgets. Billy Bob Thornton has been quoted as saying that two of his phobias are antique furniture and midgets.
Billy Wilder advised Billy Bob Thornton to write a screenplay where he could exploit his less than perfect features. After the movie launched Thornton's career, Thornton publicly discussed his conversation with Wilder, which was at a cocktail party where Thornton was working as a waiter. Thornton got a call from Wilder, who invited him over to his house. Wilder said he didn't recall the conversation with Thornton but was glad that he heeded his advice. As a gift, Wilder gave Thornton a paperback copy of the Sling Blade script with his autograph and a personal message inscribed on it.
Billy Bob Thornton requested that no red be seen in this movie stating : "I don't like to see red in a movie, unless it's about the Revolutionary War". Billy Bob decided to leave in the shot of a town fire engine driving by.
Cameo by Col. Bruce Hampton Ret. of Aquarium Rescue Unit and Code Talkers during Doyle's band scene. Col. Bruce is the Tambourine player and can be seen blowing his trademark "Smokeless Smoke Ring" in a close up shot.
Harvey Weinstein - then co-chairman of Miramax - only saw the first 30 minutes of Sling Blade (1996) when he agreed to pay $10m for the rights to the film. He later regretted this and forced Billy Bob Thornton to cut about 20 minutes from the movie.
When the film was in pre-production, Billy Bob Thornton envisioned John Ritter's character as having dyed blonde hair, thinking that Vaughan was a man who was transferred out of St. Louis for his job in where the film is set, and based his hairstyle on what he saw men sporting in GQ magazine. Thornton, Ritter and the film's hairstylist began experimenting with hairstyles on Ritter at the hotel where the cast and crew stayed. According to Thornton, Ritter was insistent that the hairstyle wasn't too drastic as he had to go back to Los Angeles to shoot a public service announcement. But when the hairstylist came up with, and applied the hairstyle that Ritter's Vaughan character would sport in the film, Thornton immediately approved. However when Ritter finally got a look at his hair in the hotel room's bathroom mirror, he was infuriated at Thornton. Thornton said that when Ritter went back home to shoot the PSA, he was wearing a baseball cap.
In his autobiography "The Billy Bob Tapes", Billy Bob Thornton had become good friends with John Ritter when they were shooting the sitcom Hearts Afire. This friendship inspired Thornton to cast Ritter as the first person in his film. According to Thornton, his first scene with Ritter was in the cafe when Vaughan (Ritter) tells Karl (Thornton) that he's gay. Thornton said that none of the film's cast had watched him in character as Karl. So when he got into character, Ritter burst out laughing, as he thought Thornton was joking. Thornton explained that it was the character, an explanation which Ritter said was going to be difficult for Ritter to act with, as he knew Thornton too well. Thornton said they were able to shoot the scene but its difficulty was due to Ritter trying to keep a straight face throughout the shoot.