NCIS Stars Confirm What We Suspected About Mark Harmon’s Behavior: — For years, fans and industry insiders have speculated about the demeanor, professionalism, and behind-the-scenes conduct of Mark Harmon, the enduring star of NCIS. Now, his fellow cast members are finally confirming what many suspected all along: Harmon is not only a consummate professional but also a quietly supportive force on set. Colleagues describe him as patient, focused, and deeply committed to creating a positive enviro