To prepare for his role as Navy submarine Capt. Joe Glass in the just-released movie “Hunter Killer,” actor Gerard Butler traveled to Pearl Harbor to spend three days aboard the Los Angeles-class attack submarine Houston.

“Hunter Killer” is the story of a submarine crew that’s tasked with stopping a Russian coup and avoiding the miscalculation that could lead to World War III. It was filmed with the Defense Department’s support; the Pentagon sees it as an effective way to connect with movie-going young men and women who might consider the military after watching the action-adventure film.

Like it is doing with “Top Gun 2,” after approving the movie script, the Pentagon made its ships, aircraft and personnel available for shooting scenes, and had advisers around to assist with set accuracy. The movie-making is done at no cost to DoD. If an action sequence can’t realistically be made part of a training mission, the Pentagon bills the production company for the time.

Butler visited the Pentagon Monday and conducted an unexpected press conference with defense reporters.

On ‘man overboard’ drills. “I don’t know if I’m allowed to say this, but when you are doing a man overboard, rather than putting a man overboard, they throw a bag of popcorn into the water,” Butler said. “Then you spend the next — you have four minutes, because if you are in cold water, he’s not going to make it, and neither is the popcorn — because, actually, the bag breaks open. So you spend the next four minutes maneuvering an 8,000-ton sub to try and get next to the popcorn so somebody can jump in and rescue it.”

He slept in the executive officer’s quarters. “I slept in the same room as the XO,” Butler said. “So when he was working, I was sleeping and vice-versa. There was two bunks in the room. Director Donovan Marsh got to sleep in the crew’s quarters.”

They were underwater for three days. “Basically, while we were down there we drilled pretty much all of the sequences that we would do in the movies. So we had battle stations. We had dives. We had quick surfaces, we had fire drills and it gave us a lot of ideas.”

That led to building a sub set on gimbals. "We ended up with a 17-ton set with 40 actors and crew and all the camera equipment basically working on this hydraulic platform. So when we go for our first dive in the movie, we could lean back, and when we were in the action sequences or torpedo chases, or avoiding depth charges ... the sub could move. We didn’t have to do the ‘Star Trek’ thing,” Butler said, which he demonstrated by stiffly doing a Trekkie-like shuffle to the side of the podium and leaning sideways.

He did not work out while on board. "But we put that in the movie,” Butler said. “There’s a little space where guys are cramped in with the bike to show the uses that have to be made of these tight spaces on the sub.”