In one episode, Gibbs is asked what Ducky looked like when he was younger. He responds, "Illya Kuryakin," who was David McCallum's character in The Man from U.N.C.L.E. 40 years earlier.
Gibbs once asked Tony for help with the crossword puzzle clue "TV Drama" and Tony answers "St. Elsewhere," which Mark Harmon appeared on.
Tony once says "I've got a better chance of hooking up with Jessica Alba than these guys do of infiltrating Sea Lift." Michael Weatherly was engaged to Jessica Alba for a time. He also starred with her in Dark Angel, where they kept trying to hook up.
In the episode Obsession (7x21), Tony obsesses over a journalist who embodies all what he finds attractive in a woman, and who is missing. When calling her at her cell phone, he says: "It would seem strange, but I feel like I know you". It's not so strange, though: the journalist is played by Ashley Scott, who also played underground operative and potential secondary love interest Asha Barlow in Dark Angel.
In the Season 4 Halloween episode "Witch Hunt", Abby, in her Marilyn Monroe Halloween Costume, calls Gibbs "Mr. President" in a reference to John F. Kennedy. However, Mark Harmon also played the US President in the 2004 film, "Chasing Liberty". That said, Abby was more likely attempting to evoke the time Marilyn sang "Happy Birthday" to John F. Kennedy.
Harmon's turn as serial killer Ted Bundy (in the TV movie The Deliberate Stranger) is referenced by Fornell in the episode "Smoked" (4x10).
Fornell: I was on the team that nailed Bundy, Gibbs. And I'm familiar with sick, charming bastards. Gibbs: That's probably why we get along so well.
In the season eight double episode "Enemies Foreign/Domestic" (also the name of an episode of The West Wing in which Harmon appeared) Tony makes a reference to The West Wing by calling McGee "McBartlet". Both Mark Harmon and Michael O'Neill (who guest stars in these episodes) played secret service agents on the show.
A combination of Actor Allusion and Shout-Out: Tony (Mike Weatherly)'s father is played by Robert Wagner. Weatherly met NCIS producer Donald Bellisario while Weatherly was filming The Mystery of Natalie Wood, in which he played Wood's husband...Robert Wagner.
In the Episode 'Light Sleeper' while on stake out Tony is reading an entertainment magazine
Tony: Damn, I can't believe this... Mick and Jessica broke up, I'm always the last to know.
This is just after Michael Weatherly and Jessica Alba broke up.
A retroactive one: Early in Season 7, a Metro detective who annoys Gibbs is described as having "tugged on Superman's cape".
In the Season 9 episode "Newborn King", Jimmy Palmer, when explaining that his future Father-in-Law, Ed Slater, is visiting the NCIS building, he mentions that the first time he learned that Palmer was marrying his daughter, "[Ed Slater] laughed, and then he cried from laughing too hard". Ed Slater is played by Larry Miller, who himself used the phrase "I laughed, I cried" during his first Is This Thing Still On? moment as Principal Jindrake in the movie Max Keeble's Big Move.
In season five's "Recoil", Nick Chinlund guest-starred as a detective hunting a serial killer who murdered women and cut off their fingers. Sixteen years earlier, he played Donnie Pfaster on The X-Files — as a serial killer who murdered women and kept their fingers.
Season 9 introduces Dr. Samantha Ryan (played by Jamie Lee Curtis) as a psychologist and love interest for Gibbs. This is a reference to Freaky Friday in which she played a psychologist, frustrated with the fact that her unruly teenage daughter refuses to accept her fiance—named Ryan, played by Mark Harmon.
Backed by the Pentagon: The Navy and USMC support the show because malfeasance (regardless of whether it is done by service members or otherwise) is always brought to justice. In "Child's Play" season 7 episode 9, NCIS Agent Ray, who lost in checkers to the child prodigy, was played by Ray Mabus, the current Secretary of the Navy.
Beam Me Up, Scotty!: Abby is repeatedly misquoted as having screamed "Stop looking up my skirt!" in "Bloodbath". What she actually says is "And don't look up my skirt!"
Pauley Perrette studied sociology and criminal science before becoming an actress and playing forensic specialist Abby Sciuto.
An interesting inversion with David McCallum, who knew nothing of pathology when he was cast as Ducky, but developed such an expertise over the years in his studies for the role to make Ducky as spot on to real coroners that the showrunners were considering paying him a consulting fee.
Fake Nationality: Coincidentally, the in-series siblings Ziva and Ari. The Israeli Mossad agent Ziva is played by Chilean actress Cote de Pablo, and the Israeli/Palestinian double agent Ari is played by German actor Rudolf Martin.
Director Thomas Morrow is portrayed by Alan Dale, who is from New Zealand.
Fandom Nod: In the season 9 episode "Life Before His Eyes," Gibbs imagines what his life would be like if several things had gone differently. In his alternate timeline, Ziva never came to join NCIS and instead gets arrested for terrorism. Tony is the one to interrogate her, and "guesses" her name wrong several times. One of the names he calls her is "Tiva", which is the online Portmanteau Couple Name given to the Tony/Ziva ship.
Hey, It's That Guy!: The main cast have appeared in a lot of other shows. Mark Harmon, Rocky Carroll and Lauren Holly all costarred on Chicago Hope. Harmon was a mainstay on St. Elsewhere. Carroll had his own sitcom and was the voice of Derek Maza/Talon on Gargoyles. Holly had starring roles in Dumb and Dumber and Picket Fences. Sean Murray was on Hocus Pocus and JAG; he played two different characters in JAG so if you count it as being in the same verse as NCIS it means he's played 3 different characters in the same universe. Michael Weatherly was on Dark Angel. More recently, the actress who played Agent Lee, Liza Lapira, has turned up in Dollhouse and Dexter. After Caitlin Todd died, she morphed into Maura Isles. Ducky co-starred in The Man from U.N.C.L.E. as Illya Kuryakin (or as Eric Ashley-Pitt from The Great Escape). NCIS's pilot episode can count as a Mythology Gag of sorts, since Ducky at times appears to be the team leader instead of Gibbs, mirroring his prominent role in UNCLE. See also Actor Allusion. Also, although not likely to be brought up in an American show, McCallum was the Steel half of Sapphire And Steel.
Stargate SG-1 fans will wonder about how the SGC intends to get Jonas Quinn off for serial murder in "Skeletons". They might be further confused by Martouf being a dirty Metro cop in "Lost And Found". Master Bra'tac apparently has a double life as elusive ultra-forger Khamal Khakani. Colonel Makepeace appears as an ex-Army colonel running a mercenary band that's hunting Mike Franks' daughter-in-law in a season 7 episode. Valla murders a man who has been killing off her dealers in season 7 episode "Borderland". The late Don S. Davis appeared as a government official in the second season and contributed to one of McGee's most awesome moments. And Jack O'Neill's clone was apparently adopted by a Marine and who was forced to hold a classroom hostage by Mexican drug runners. Stargate Atlantis fans will notice that Michael kidnapped Director Sheppard, and Elizabeth Weir is an overworked ME in "Identity Crisis" and recurs as Ducky's temporary replacement. For the Stargate Universe fans there's the deceased Gloria Rush as the now deceased Lara Macy and Tamara Johansen as Dana Arnett from the season 5 episode 'Leap of Faith'.
One episode had a random teenager played by Zac Efron, in all his feather-headed glory.
Zain Nadir was a Mossad agent who Ziva slept with.
Season 8 had a guest-appears of Alimi Ballard as Special Agent Levin, that is, David from NUMB3RS.
The teenager who holds his class hostage with a bomb in Season 3 is Mike Newton.
Peter Burke plays one of Gibb's old friends in "Capitol Offense." He didn't kill the first person but did help cover the crime up with another killing. This one is especially surreal for fans, since the character cheated on his wife, an act which is pretty much as likely to happen on White Collar as Mozzie making gift baskets for the suits.
River Song has apparently taken up black market dealings and flirting with Gibbs.
Gibb's father? John Walton. No wonder he's good with a shot.
The grief counselor brought in for the aftermath of the season 9 finale/season 10 premiere was Alistair.
The Pete Best: Robyn Lively played Special Agent Vivian Blackadder in the two-part JAGPoorly Disguised Pilot. Donald Bellisario reportedly said she was "too soft" for the new show, and she was replaced by Sasha Alexander's Caitlin Todd for the series proper.
Sean Murray, who plays McGee, is the real-life stepson of the show's creator Donald Bellisario.
McGee's sister is played by his real-life stepsister.
In one season 9 episode, Abby shows the others a locket containing a picture of her mother. The picture is of Pauley Perrette's real-life (deceased) mother, Donna. The lock of Abby's mother's hair inside the locket was also that of Pauley's mother.
The young Gibbs is played by Mark Harmon's son Sean.
Sleeper Hit: NCIS is one of the rare examples of a show that's steadily increased its viewing audience as it's gone on, and to continuously do it over a decade is virtually unheard of; nearly every successive season has had an episode that set a new benchmark for the show's viewershipnote with only seasons 4 and 5 unable to turn the trick, with Season 10's "Shiva" the current record holder at over 22 million viewers.
Troubled Production: When Donald Bellisario was showrunner, he exercised tight control over the show and continuously rewrote scripts, resulting in the cast and crew working longer and longer hours because of the resulting delays. While this isn't unusual in TV production, Mark Harmon finally decided to put his foot down and threatened to quit unless someone else was brought in as showrunner. While this smoothed out the production process, Bellisario is now in a messy and ugly legal battle with CBS over royalties that's discouraging the network from pursuing crossover opportunities between the two NCIS series currently on the air.
When Cote de Pablo's contract ran out after Season 10, she entered into protracted negotiations. While the powers that be said they kept on increasing their offers, de Pablo was hesitant to accept and dragged talks out through the hiatus before deciding to leave a week before production on Season 11 was supposed to start. This left the writers with little time to put together a workable departure storyline fox Ziva and cast members publicly expressed displeasure at how the actress left (Mark Harmon being particularly backhanded during an appearance on Arsenio Hall).