A good deal of the “The Presidents’ Gatekeepers,” a documentary-style look at the stiflingly high-pressure world White House chiefs of staff inhabit while serving as POTUS’ right-hand guy, is dedicated to the somber reflections and gut-wrenching decisions administration aides had to help make during America’s darkest hours.

Luckily, spirits were much higher (open bars always help) at Tuesday’s preview party.

The two-part series, airing at 9 p.m. Wednesday and Thursday on the Discovery Channel, features candid discussions with the 20 living ex-presidential confidants, as well as commentary from Presidents George Bush and Jimmy Carter.

The screener focused on some of the most harrowing moments in recent history, taking the participants back in time to expound on their roles during trying times such as the 1981 assassination attempt on President Ronald Reagan, the atrocities of 9/11 and the 2011 raid on Osama bin Laden’s compound.

In the film, Andrew Card says he was as stunned as everyone else by the back-to-back assaults on the World Trade Center towers. But the aide to President George W. Bush insists he deliberately opted against causing a scene during the fateful morning “Dubya” was shown reading to schoolchildren in a Florida classroom.

According to Card, he leaned in, whispered, “America is under attack” to the visibly rattled Bush and then withdrew so 43 couldn’t engage him further.

“It wasn’t the place to have a conversation,” Card said.

Vice President Dick Cheney immediately switched into survival mode, ordering the armed forces to ground any potentially threatening aircraft — including the hijacked plane heroic passengers would eventually bring down in Pennsylvania — still circling the skies. Cheney was chief of staff to President Gerald Ford.

“Flight 93 … was a weapon aimed at the heart of America,” Cheney explained of his shoot-first-ask-questions-later edict.

Flash-point decisions, it would seem, have always been the bane of the Oval Office staff’s existence. But many of those interviewed said the onus is always on keeping the boss as well-informed as humanly possible.

Jim Jones, chief of staff to President Lyndon Johnson, said he learned early on to do as much homework as possible before ever bending the president’s ear.

“Hold off that decision as long as possible, because there’s always new information coming in,” he told HOH, advice that continues to ring true given the swirling situation in Syria.

Jack Watson, chief of staff to Carter, urged politicians of all stripes to always keep their options open.

“You need to be careful not to make unnecessary enemies,” he counseled. “[Because] the person that’s with you today could be against you tomorrow. And vice versa.”

Ken Duberstein, chief of staff to Reagan, hinted that the most important trait for a chief of staff to cultivate is humility.

“The binding thing is all of us are staff, not chief,” he quipped.

Joshua Bolten, chief of staff to George W. Bush, said he most enjoyed the creature comforts: decompressing at the Bush ranch in Crawford, Texas, (“Just hanging out with the president and his family was such a joy.”) and escorting the family dog, the late Barney, all around the White House grounds. (“It was a joyful part of the job.”)

He was also shocked to learn of Dubya’s budding art career. “I never saw him pick up a brush when I worked for him,” Bolten shared.

And while Discovery Channel should be commended for rounding up the living legends, this was hardly the first time the professional family has gotten together to swap stories.

Bolten said he invited the 16 living chiefs of staff he knew about to the White House in late 2008 to give incoming top administration aide Rahm Emanuel some pointers.

Thirteen ex-chiefs, including Cheney and Donald Rumsfeld, showed for the breakfast, and the group spent more than two hours dredging up fond memories and dispensing hard-earned advice. Bolten said he was pleased by the attendance — and even more so by the fact that Emanuel was “uncharacteristically in listening mode.”