You’ve likely seen a thousand action movies before Netflix’s Triple Frontier. What you might not have noticed, however, is how actors handle their prop guns. For very obvious and reasonable safety reasons, blanks are used instead of live ammunition during pre-production for many action movies.

That did not happen in the making of Netflix’s newest movie, Triple Frontier.

The heist thriller, now streaming on Netflix, notably trained its star-studded cast — including Ben Affleck, Oscar Isaac, Charlie Hunnam, Garrett Hedlund, and Pedro Pascal — with live ammunition instead of blanks. The filmmakers say this made an important difference in how the actors behaved while filming the movie’s pulse-pounding scenes.

“If you actually look at what these characters are doing, it’s kind of an extraordinary combination of skills and emotions,” director J.C. Chandor said in a roundtable interview ahead of the film’s streaming release. “I’m not a huge rehearser. But the rehearsing, I think, helped with all of them was live fire training. Which means it’s real bullets.”

In Triple Frontier, a group of US military veterans languishing in their civilian lives band together to rob a Colombian drug lord. Because the soldiers are acting outside any official sanction, the stakes are especially high to not only survive but also not get caught and court-martialed.

Netflix's 'Triple Frontier', now streaming on Netflix. Netflix

While not “super soldiers” like in a sci-fi movie, the characters of Triple Frontier are meant to be some of the most talented soldiers ever in the field. But to sell the part, the real soldiers who were hired to train the stars encouraged giving them guns with live ammunition all throughout their two-week training period in California’s Simi Valley.

“We worked with these great trainers and tactical advisors,” said Ben Affleck, who stars in Triple Frontier as “Redfly,” the operations leader of the heist. “We shot live weapons and tried to learn how to emulate them as best we could.”

“It was amazing. I was shocked by how much trust they put in us,” said Charlie Hunnam, who plays Captain Miller. “Very, very quickly, they allowed us to be on the range with live fire, doing increasingly complex maneuvers. We started ambush scenarios, shooting through windows and panes of glass, doing cover fire, and operating movements I’ve never done before.”

Two Special Forces advisors, identified as Nick John and Kevin Vance, were “the most important part of this whole thing,” said Oscar Isaac (“Pope”).

“Nick John was with us every step of the way, just building our confidence and help us take these things seriously. Focus on what really matters. Not just how many push-ups we can do but how to think in a tactical way.”

He added, “By the end of the first day, they gave us live ammunition and trusted us with that, which seemed a little irresponsible.”

Garrett Hedlund and Charlie Hunnam in Netflix's 'Triple Frontier'. Netflix

“Most training sessions happen with false ammunition,” explained Chandor. “They were all shooting real bullets. It was the military guys’ suggestion. When you look at someone that’s been trained to fire a gun without a bullet in a movie, you can tell in a second because the gun will ‘swipe’ across you. Of course, there’s no risk. You’re trained in a different way. If you train for two weeks with live ammunition, if you put that anywhere near your leg, you’re gonna know it because you don’t want a bullet in your leg.”

Training with real bullets didn’t intimidate the actors. But swimming with mules? That actually did.

About halfway through the movie, the soldiers try to carry millions of dollars through South America and have to swim across a river with mules. The problem: Well, mules kind of like to relieve themselves in large bodies of water.

“When a mule enters water, he doesn’t want to be there and swims as fast as he can to get to the other side,” explained Hedlund. “J.C. came to Charlie and me and said, ‘Would you be willing to swim with the mules?’ These idiots don’t because they might poop or pee in the water.” Hedlund and Hunnam, reluctantly, agreed. “He went to them and said, ‘You’re off the hook.’”

Between training with real bullets and swimming with mules, the off-set bonding of Triple Frontier for its actors helped sell the film’s emotional core, that you could believe these men would die for each other in the most trying of circumstances.

Ben Affleck, with the mules, in Netflix's 'Triple Frontier'. Netflix

“It came from spending time together,” said Affleck. “As a bunch of actors I really like and respect made it really easy for me. We all sort of knew that the movie would fall apart without some sense of history between these people. I think everybody was invested in that.”

“What I’ve found with casting — thankfully and hopefully it continues —the people who end up being there on principal photography are usually the people who are meant to be there,” said Chandor, who has seen a number of actors come and go before Triple Frontier began production. “They’ve had enough passion and believed in something enough to show up and be there.”

Triple Frontier is now streaming on Netflix.