'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.

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 friendship.