1. Actor David McCallum who plays Ducky and was nominated for 3 awards in his 5-year run on “The Man from U.N.C.L.E.” On NCIS, this was referenced on S:2 EP:13 at the 21:40 mark when Kate asked Gibbs what he thought Ducky looked like as a young man.
2. Mark Harmon, who plays Leroy Jethro Gibbs. He has his real life son Sean Harmon play his younger self in any flashback scenes.
3. Michael Weatherly who plays Tony Dinozzo. He comes from a wealthy family. But he and his real life father are estranged. His father (played by Robert Wagner) are on the show.