You know your TV show is popular when big-time, bankable Hollywood stars make guest appearances. The CBS drama  is so marketable that  acted on the show over the years.  is off the charts thanks to his starring role. We know the show is must-see TV for most people, but how real is ? This is what real-life NCIS agents have to say.

What do real NCIS agents do?

| John Althouse/AFP/Getty Images

The Naval Criminal Investigative Service has naval in the name, but it’s actually a civilian agency. Agents do many of the same things as police detectives, but with a focus on the Navy and Marine Corps. However, the agency’s director reports to the Secretary of the Navy.

Real world NCIS agents have four areas of expertise, according to the :

Criminal investigations: Investigating crimes involving Navy and Marine Corps service members is the bulk of the job for real NCIS agents. They also investigate crimes affecting either military service.

How real is NCIS, according to agents?

Some of what TV’s  agents do is close to what real-life agents do. | CBS

As with any TV show,  spices things up in the name of drama and entertainment. However, a lot of the nitty-gritty on the show hews pretty close to what agents do in real life.

“Agents with NCIS will work counter-terrorism, protective operations, general crimes, economic crimes, fraud,” NCIS special agent Kevin Dodds tells .

“ is very realistic of what we do,” special agent Bob Milie says.

All the various iterations of  you see on TV hit the bullseye when it comes to portraying agents’ dedication to the job. The passion for the job and the way agents carefully investigate crimes are the same on TV as they are in real life.

Where does the show get it wrong?

 strives for authenticity, but there are a few big differences between TV and real life.

The loving head slaps Mark Harmon’s character does on TV’s  don’t in real life. | CBS

For one thing, there aren’t a ton of struggles over jurisdiction. Real-life NCIS agents know their boundaries, and they work with other agencies if they need to cross those boundaries during an investigation.

Also, there isn’t an Abby, Pauley Perrette’s character before she , waiting in a lab to analyze forensic evidence. The same goes for the show’s medical examiner, Donald “Ducky” Mallard. There were real-world NCIS agents did forensics analysis when the show started, but an Army lab takes care of those duties now. Some agents do handle dead bodies, but NCIS has never had a medical examiner dedicated to doing autopsies.

Mark Harmon’s Leroy Jethro Gibbs character enjoys affectionately slapping fellow agents on the back of the head, but that never happens in real life. If a real agent did that, it would lead to disciplinary, according to .