‘NCIS’ star Mark Harmon says real agent shaped how he played Gibbs Mark Harmon and retired NCIS special agent Leon Carroll Jr. wrote a new book, "Ghosts of Sicily." They spoke to Fox News Digital about their decades-long friendship. NEW You can now listen to Fox News articles! Mark Harmon spent years chasing criminals on "NCIS"— but it was an off-screen encounter with a real agent that truly cracked open a turning point in his career. The actor first met retired NCIS Special Agent Leon Carroll