Mark Harmon, known for his role as Special Agent Leroy Jethro Gibbs on "NCIS," was a notorious prankster on set. However, he never played any pranks on his co-star David McCallum, who played Ducky on the show, out of respect for him. Harmon has always admired McCallum's work ethic and experience in the industry. Following McCallum's passing, Harmon and other "NCIS" co-stars expressed their condolences and highlighted the impact McCallum had on their lives.