According to Christopher Lloyd, after filming the scene where his character spears Gunther's apple with a knife and eats it, Hank Johnston, the young actor playing the role, was so frightened of Lloyd that he wouldn't approach him afterwards on the set.
During his first audition, Mason Gamble pulled a live worm out of his pocket. This spontaneous move inspired the casting directors to cast him.
Over 20,000 little boys auditioned for the titular role of Dennis Mitchell. Ten were selected to test the role with Walter Matthau.
Christopher Lloyd was previously offered the role of burglar Harry Lyme in Home Alone (1990), which John Hughes also produced and wrote. Lloyd declined to play the role, which then went to Joe Pesci. Lloyd regretted his decision to decline the film after its massive success, and told Hughes he was willing to appear in another subsequent film for him. Hughes then wrote the role of Switchblade Sam specifically for the film in hopes that Lloyd would play the role, and Lloyd immediately accepted.
Jeannie Russell: One of the Mitchells' neighbors. Russell had originally played the role of Dennis Mitchell's nemesis Margaret Wade on the classic TV series Dennis the Menace (1959).