Sim, de acordo com este artigo da MentalFloss
- THORNTON PUT CRUSHED GLASS INSIDE HIS SHOES.
It gave him Karl’s famous limp.
Também corroborado por Chron (um jornal independente de Austin, TX) em este artigo
It hurt to play Karl Thornton told James Lipton on "Inside The Actor's Studio" that playing Karl was physically painful. His face had to be contorted and Thornton put broken glass in his shoes to get Karl's tortured walk down
Por fim, a revista Esquire no Reino Unido publicou o seguinte artigo
Billy Bob Thornton Thornton took some extreme steps indeed for his role of a partially handicapped man in the sleeper hit Sling Blade. In an effort to maintain the consistency of his character’s memorable shuffle throughout filming Thornton placed real crushed glass inside his shoes, resulting in a painful limp. He was repaid with an Academy Award nomination for his pains. Unlike Lewis though he didn't get the gong - which must have been devastating.