Pauley Perrette, who plays Abby, actually graduated from college with a degree in Forensics, and then moved to New York to get her master's, and it was there that she got in to acting, before coming to NCIS. Producers have said that most of the personality and interests displayed by Abby come from Perrette just playing herself.
In one episode Kate asks Gibbs "Gibbs? What did Ducky look like when he was younger?" Gibbs answers "Ilya Kuryakin". Ilya Kuryakin is the role David McCallum played in the show The Man from U.N.C.L.E. (1964).
The sound heard at the beginning and end of each act, when the scene goes to black and white is Donald P. Bellisario making that noise into a microphone. He wanted a unique way of bookending each act and took a microphone and went into a studio and made that sound into the microphone.
In 2005 Mark Harmon requested that nobody make any fuss about his September 2nd birthday. New cast member Lauren Holly had other ideas though. The frequent victim of Harmon's practical jokes, she collaborated with Michael Weatherly on a Harmon birthday extravaganza. First they ordered 150 t-shirts that read 'It's Mark Harmon's birthday', that were passed out to the entire crew. On Thursday, September 1st, she waited in her hotel room for the heads-up call from Weatherly that shooting was wrapping. At three o'clock a.m., still in her pajamas, Holly got the call, loaded up her car with party supplies and drove back to the show's sets in Santa Clarita. She and Weatherly then proceeded to cover Harmon's trailer with crepe paper, helium balloons and piñatas, then dumped large amounts of confetti everywhere. They decorated every wall, truck and trailer, then, with the cooperation of the art department, put up a picture of Harmon on all the plasma-screen displays in the squad-room set. Holly then hopped on a plane on Friday to fly back to her husband and three sons in Chicago and waited for word on Harmon's reaction. When Harmon arrived to work he saw everyone in identical t-shirts, holding signs, and assumed there was a strike. Weatherly was called by the nervous producers, who were worried what Mark would think, and who was behind it. Michael replied "I'm not going to name any names. Lauren Holly". Harmon was very touched by the gesture, which was filmed so he could take the DVD home to his wife. He did however tell Lauren with a grin, "Payback's a bitch".
The t-shirts that Gibbs wears while working on the boat in the basement of his home are sent to Mark Harmon from law enforcement agencies around the country. He also occasionally wears a sweatshirt with the letters "NIS". This stands for "Naval Investigative Service", the predecessor of the NCIS. An agent with Gibbs' seniority would have begun his career working under that name.
Donald P. Bellisario considered casting Scott Bakula (who starred in his previous television production, Quantum Leap) as Gibbs. Eleven years later, Bakula was cast as Special Agent Dwayne Pride, Gibbs's former partner and star of NCIS: New Orleans.
Mark Harmon, who was born in 1951, would be old enough to be a Vietnam era veteran, but his character of Gibbs stated that he was a Gulf War veteran as he was too young to be a Vietnam vet, so Gibbs is much younger than Harmon.
In the opening credits of the first couple seasons, an aircraft carrier is seen going under the Golden Gate Bridge. This is a clip from the opening credits of The Presidio (1988), starring Mark Harmon as a San Francisco police inspector investigating a murder on the Presidio army base in San Francisco.
Donald P. Bellisario confirmed that before Sasha Alexander decided to leave the show, his plan for Season 3 was to build on the Tony/Kate relationship, and the ideas for the episodes NCIS: Under Covers (2005) and NCIS: Boxed In (2006) (which have a large emphasis on the Tony/Ziva relationship) were conceived with the intention of forcing the characters to confront their emotional attachments to the other.
Harrison Ford, who was experiencing a bit of a slump around the time of the series's conception, was urged by colleagues to pursue the role of Gibbs. He did not, but Mark Harmon's Gibbs does allude to this in the first episode, name-dropping Harrison Ford and his film Air Force One.
On season 2 episode 10, Gibbs keeps getting phone calls from people asking about a Volkswagen Bug for sale. Frustrated, Gibbs asks,"Do I look like someone who would drive a Bug?!" In "The Deliberate Stranger" (1986), Mark played serial killer Ted Bundy, who drove a VW Bug.
During one episode, Gibbs was trying to solve a crossword puzzle. As his character is rarely in step with pop culture, he asked DiNozzo for a TV drama with eight letters. Tony answers St. Elsewhere (1982), which has more than eight. Mark Harmon played Dr. Bobby Caldwell on that show from 1983 to 1986.
MTAC stands for the Multiple Threat Assessment Center. Previously the Antiterrorist Alert Center (ATAC), it was changed in 2002 in response to its expanding role after the terrorist bombing of the USS Cole in Yemen and attacks of September 11th, 2001.
During the opening credits there is a note about what NCIS represents and it reads: The Naval Criminal Investigative Service (NCIS) is a worldwide federal law enforcement organization whose mission is to protect and serve the Navy and Marine Corps and their families.
The pilot for this series aired as a two-part episode of JAG (1995) in the spring of 2003. Two major changes were made for the series. A gimmick in which Ducky is shown talking to corpses - from the corpse's point of view - was dropped, and a character named Viv Blackadder (played by Robyn Lively) was replaced by the character played by Sasha Alexander.
The Dodge Chargers used by these Navy Department agents are a color called Deep Water Blue, a special color costing several hundred dollars more than a standard black or white car, which a federal agency would most likely choose.
David McCallum appeared in every episode until Season 6, Episode 23, "Legend Part II". Even though he plays the medical examiner, this included shows where people weren't even murdered. In those episodes, Ducky would consult on the psychology of the case, or offer personal advice.
In season 2, episode 2, McGee and DiNozzo are in a hotel room where DiNozzo is watching an old rerun of The Adventures of Ozzie & Harriet (1952). Ozzie and Harriet were Mark Harmon's sister Kristin's in-laws in real life.
According to Pauley Perrette on the 'tour of Abby's lab' extra on the seasons 8 DVD, Bert "the Farting Hippo" has to be locked up at the end of shooting because extras and production interns have tried walking off with him before.
The show was originally entitled "Navy NCIS". They dropped the "Navy", which was redundant because the "N" in "NCIS" stands for "Naval" referring to the Navy. "NCIS" stands for "Naval Criminal Investigative Service".
"FLETC" (flet c) is often referred to in regard to trainees. The word is an acronym for the Federal Law Enforcement Training Center. NCIS, along with 82 other federal agencies, train recruits at FLETC-Glynco, just north of Brunswick, Georgia.
Multiple times, Gibbs would comment that a shooter would "police their brass". This is a sniper's term for picking up expended casings off the ground, and is in line with Gibbs' background as a Marine sniper.
Gibbs was a Gulf War veteran, but the tee-shirt he occasionally wears could be a reminder of his NIS origins. Although the NIS was renamed in 1985, becoming the NSIC, then the NISC, it was only in 1992 the NCIS was taken out of the uniformed navy's control and a civilian director appointed.
Although it is said that the show ("NCIS") takes place in the same universe as "JAG" and "Hawaii 5-0", actor Terry O'Quinn (Lost (2004)) guest stars in multiple episodes of all three shows as three different characters with different names.
In numerous episodes someone is told to "watch your 6". During World War II pilots would call-out the positions of enemy airplanes by referring to their bearings via the use of a pretend clock face. Twelve o'clock meant the enemy was directly ahead, whereas 6 o'clock would mean directly behind. Basically this means "watch your tail". Another comment "I've got your 6" means "I've got your back".
In Season 4 episode: Shalom, as Gibbs is about to look in an air vent, another person (not supposed to be in the shot) is visible. It is obvious that this is a goof because the only characters that are supposed to be in the scene are Gibbs and Ziva.
The scene at the start where the NCIS car is seen driving away from the camera is actually out of the JAG episode "Ice Queen" (s8 Ep20) and is not an NCIS scene. Although the "Ice Queen" was later aired as part of "Navy NCIS: The Beginning"
The trivia items below may give away important plot points.
Many of the characters' histories are taken from the actors' real lives. For instance, Michael Weatherly's father has a large fortune from a family business, as does Tony's father (and Weatherly's Dark Angel (2000) character). Abby Scuito states she used to go through old cars to see what happened to them and try to find out what went wrong, which Pauley Perrette did in her childhood.
Ralph Waites, who played agent Gibbs's father in the series passed away in 2014. The final episode in season 11, in which Gibbs must deal with his father's death, was dedicated to his memory. Furthermore, in the final scene of the next episode (episode 1of season 12), the mug on Gibbs's work table has the name Ralph on it.
Season 10, Episode 18 Vance is escorting a potential Nanny to the elevator. She speaks in A British accent, is named Maria, and gives, nearly verbatim, part of the speech given by Julie Andrews in the film version of The Sound of Music while applying to be the governess for Captain von Trapp's children. After she gets on the elevator and the door shuts, Vance references "a problem" with Maria, which is a clear reference to the song in The Sound of Music titled, "A Problem with Maria."