For nearly two decades, NCIS has entertained and thrilled on CBS with its nail-biting cases. The team led by Special Agent Leroy Jethro Gibbs has tackled anything and everything you can imagine. Any criminal investigation that has to do with Navy or Marine Corps personnel falls to the Naval Criminal Investigative Services and Gibbs’ charismatic agents. A spin-off of JAG, it’s entering its 17th season and is still just as popular as ever.

These days, the team is a little different, with only Donald “Ducky” Mallard the medical examiner, MIT grad Timothy McGee, and Special Agent Gibbs (Mark Harmon) left from the original cast. Though the irrepressible Agent DiNozzo, fiesty Agent Ziva, and lovable forensic scientist Abby have all left the show, they were a part of the series for over ten years and have a place in the hearts of all NCIS fans. Here are 10 hidden details everyone missed about the main characters of one of the longest-running series on prime time!

GIBBS MAKES ALL THE REFERENCES TO MARK HARMON

Mark Harmon plays Special Agent Leroy Jethro Gibbs, the no-nonsense leader of the NCIS team in Washington. Harmon was everywhere in the 80s and 90s, and the series frequently has references to his former projects. In the episode “Chained”, Gibbs keeps getting calls about a VW Bug for sale. Exasperated, he asks the caller, “do I look like someone who would drive a Bug?!” This references the time he played serial killer Ted Bundy in The Deliberate Stranger.

In “Blowback”, Gibbs is trying to solve a crossword puzzle. He asks Agent DiNozzo if he knows the name of a TV drama that only has eight letters. Tony response, “St. Elsewhere,” a reference to the series Harmon starred in as Dr. Bobby Caldwell from 1983-1986.

PAULEY PERRETTE WAS BASICALLY ABBY IN REAL LIFE

As lovable and stylish forensic scientist Abby Sciuto for 15 seasons, Pauley Perrette got to put her real life forensic experience to the test. With a bachelors degree in psychology from Valdosta State Universe in Georgia and a degree in criminal sciences from the John Jay College of Criminal Justice in New York, she had the appropriate background to give Abby real authenticity.

In New York she discovered her love for acting, and put her Masters degree on hold. Most of her interests, and even aspects of her style of alternative fashion, got worked into Abby’s character on the show.

THE DINOZZO HEAD SLAP WAS REAL

Michael Weatherly was recently single when he started playing Tony DiNozzo on NCIS. At 31, he was separated from his former fiancee Jessica Alba, and had developed a reputation as a ladies’ man on set. Supposedly, during the filming for an episode, he was chatting up a young female extra, and apparently co-star Mark Harmon didn’t take too kindly to what was happening, and stepped over to where the two were talking.

He hit Weatherly on the back of his head, which the producers liked so much they kept it in the scene. Later on, it would become a defining aspect of Gibbs and Tony’s relationship, appearing throughout the series whenever Tony said something Gibbs found offensive.

MARK HARMON IS BASICALLY GIBBS

Though Mark Harmon is said to have a sunnier personality than his tough-as-nails alter ego Special Agent Leroy Jethro Gibbs, there are many aspects of his personality that were worked into Gibbs’ character on screen. For instance, Harmon’s father was a real-life WWII  US Army Air Force pilot, and so is Gibbs’ on the show.

Harmon got interested in carpentry when he was a young man, and his woodworking hobby was passed on to Gibbs. In fact, most of the tools that Harmon uses in the scenes where he’s working on building boats down in his workshop are all his.

COTE DE PABLO TRAINED TO BE A LINGUIST LIKE ZIVA

As Mussad officer Ziva David, important liaison to NCIS, Ziva’s background as an expert in half a dozen languages makes her invaluable to the team, as do her impressive combat skills. Playing an Israel special forces veteran wasn’t easy, and forced Cote de Pablo to become somewhat of a linguist.

Not only did she work with a Hebrew dialect coach to nail the scenes where Ziva needed to speak Hebrew, she also trained to be able to rattle off other languages as well. Ziva was known for being something of a prodigy when it came to languages, so de Pablo had her work cut out for her.

TON’Y FAMILY FORTUNE FIASCO

Special Agent Tony DiNozzo has never had the best relationship with his family, especially in regards to his father, who never approved of his career. This is also very similar to the character he played on Dark Angel opposite Jessica Alba, Logan Cale. Both characters were cut out of their family fortunes when they pursued their own life paths.

This directly relates to actor Michael Weatherly’s own personal family history. His father cut him out of the millions that he made importing Swiss Army Knives when Weatherly decided he was going to be an actor. The bitterness and resentment between the two men gives the scenes between Tony and his onscreen father so much emotional weight.

DUCKY AND THE MAN FROM U.N.C.L.E.

David McCallum played medical examiner Donald Mallard on the series since it began, offering his professional (and personal) advice to the team on everything from psychological evaluations to autopsies. He’s known for his dry sense of humor and his amusing bowties, often with eccentric patterns (red and gold elephants) or curious colors.

In one memorable episode, Kate asks Gibbs what Ducky looked like when he was younger, to which Gibbs responded, “Illya Kuryakin.” This is a reference to The Man from U.N.C.L.E. in which McCallum played agent Illya Kuryakin, a role reprised by Armie Hammer in the 2015 film starring Man of Steel’s Henry Cavill. Mark Harmon (Gibbs) often called him “Illya” on set.

GIBBS’ GARMENTS

When he’s not playing the part of Special Agent Leroy Jethro Gibbs, Mark Harmon is known for organizing charity events, doing toy drives to help Naval families, and making appearances for foundations that provide aid to veterans. This has made him a big hit with his fanbase, who have taken to sending him old t-shirts from the armed services to him as gifts.

Whenever Gibbs is down in the basement of his home working on his boats, he’s often seen wearing t-shirts from the Army, Navy, Marine Corps, and Air Force. At first, these were found by the wardrobe department to add authenticity, but with the help of the fans, all the t-shirts became provided.

RECYCLED FOOTAGE

In the first few seasons of NCIS, footage was shown of aerial views of an aircraft carrier under Golden Gate Bridge. This is actually footage from the 1988 film The Presidio, an intense cop thriller starring Mark Harmon. Harmon plays ex-military policeman Jay Austin, investigating the death of his former MP partner at a military base.

It wasn’t the last time Harmon would play a former military character. In The West Wing, he played Agent Simon Donovan, a role which garnered the interest of Donald Bellisario when he was looking to cast the character of Special Agent Gibbs.

THEIR DESK DECOR

If you’ve ever paid attention to the desk decor in the squad room, you’ll notice that the personal tastes and hobbies of the main characters find their way into their professional lives. If you look closely at McGee’s desk, you’ll see posters for the electronic group Venetian Snares, a band that Sean Murray happens to love as well.

Ellie, who joined the series in Season 11, has a desk area plastered with Oklahoma State University spirit gear, from pendants to class pictures. Actress Emily Wickersham was a student there, and wanted a way to give her alma mater a little shout out.