Mark Harmon starred as Leroy Jethro Gibbs on NCIS for eighteen seasons and also served as executive producer of the show. In other television work, Mark received an Emmy nomination for Outstanding Guest Actor in a Drama Series for the critically acclaimed The West Wing and for Outstanding Supporting Actor in a Drama Special for Eleanor and Franklin: The White House Years. Other credits include Reasonable Doubts, for which he received two Golden Globe nominations; Chicago Hope; From Earth to the Moon; St. Elsewhere; and Moonlighting.

Leon Carroll, Jr., a native of Chicago, Illinois, was a commissioned officer in the United States Marine Corps, attaining the rank of major. Leon then began a twenty-year career as a special agent with the Naval Criminal Investigative Service (NCIS). He served in seven different locations, including tours as a Special Agent Afloat on the USS Ranger(CV-61) and as the Special Agent in Charge of NCIS offices in the Republic of Panama and the Pacific Northwest. Upon retirement from NCIS, Leon was selected to be the technical advisor for the top-rated drama of the same name, holding that position for twenty seasons.