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.
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 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.