Michael Weatherly is the only cast member Donald Bellisario allows to ad-lib lines.
Since the two-part episode "Legend" is also technically part of NCIS: Los Angeles canon, it makes Gibbs, Tony, Abby, and Ducky the only characters who have appeared on the two NCIS shows and its parent series JAG. Abby is the only one who also appears in a stand-alone NCIS: LA episode.
Season 6, "Agent Afloat": Maybe real aircraft carriers posing as fictional ones shouldn't let viewers see their registration numbers. When this episode aired, producers heard from several "carrier purists" complaining that CVN74 is the U.S.S. John C. Stennis, not the (non-existent) Seahawk.
There is an episode where someone is killing people in the same ways McGee describes in his yet unpublished next book. Over the course of the investigation, they question McGee's book publisher. Along the wall of her office, there are pictures of all the authors that the company publishes. Show creator Donald Bellisario is pictured as one of those authors.
The show originally premiered as "Navy: NCIS", and ran with that title for the first season. Beginning with season 2, the title was shortened to just "NCIS", after the producers convinced CBS (the network had insisted on the longer title) that "Navy: NCIS" was redundant, since the "N" in "NCIS" stands for "Naval".