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.
Whenever we see flashbacks featuring young Gibbs, the younger version is played by Mark Harmon's son, Sean, except in the NCIS: Origins series.
Donald P. Bellisario considered casting Scott Bakula (who starred in his previous television production, Quantum Leap (1989)) as Gibbs. Eleven years later, Bakula was cast as Special Agent Dwayne Pride, Gibbs's former colleague and star of NCIS: New Orleans (2014).
The hand tools that Mark Harmon uses on the set of NCIS are mostly his own. Those are the tools that he uses in his own personal workshop. His wife, Pam Dawber, once stated that he will get on her case if the tools in the workshop are not put back exactly as they were.