Ja, enligt den här MentalFloss-artikeln
THORNTON PUT CRUSHED GLASS INSIDE HIS SHOES. It gave him Karl’s famous limp.
Också bekräftad av Chron (En oberoende Austin, TX-tidningen) på den här artikeln
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
Slutligen skickade Esquire i Storbritannien följande artikeln
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.