Two days before production began on 'Lone Survivor,' former Navy SEAL Marcus Luttrell surprised the cast and crew by walking unannounced onto the Albuquerque SWAT rifle range, dialing in a 5.56x45mm Mark 12 rifle, and firing at a target. The film's director, Peter Berg, was putting the actors through live-fire training in the high-desert mountains – a stand-in for Afghanistan's Hindu Kush, where in 2005 Luttrell was the only member of a four-SEAL team to survive a Taliban ambush. (Sixteen soldiers attempting to rescue them in a helicopter also died.) It was one of the deadliest attacks against the SEALs ever, and even Berg wasn't sure to what extent Luttrell, 38, would be up to reliving the nightmare to help the film. "I'm not an actor," Luttrell says, "but I wanted to ensure the story is told properly, without exaggerations."

Berg, a seasoned filmmaker best known for 'Friday Night Lights,' would come to lean heavily on Luttrell during the production. "I've never felt more pressure to get a film right," Berg says. "The SEAL community told me pretty aggressively, 'You'd better not fuck it up.'"

More than any other war movie, 'Lone Survivor' conveys the gritty intensity of a small-unit Special Forces team in battle. The film recounts the two-hour firefight that pitted Luttrell's team against some 100 armed Taliban fighters. In June 2005, Luttrell and three fellow SEALs – Lieutenant Michael Murphy, 29, and petty officers Matthew Axelson, 29, and Danny Dietz, 25 – were dropped 20 miles behind enemy lines in the snowcapped mountains of northeast Afghanistan's lawless Kunar Province. Warned of the SEALs' presence by local goatherds, the Taliban ambushed them, killing the soldiers as they fought their way down the mountain until Luttrell was the last man standing. Near death, he was saved by Mohamad Gulab, 39, a herder who sheltered the SEAL and engineered his escape. Back home in Marquez, Texas, Luttrell wrote a 2007 memoir, 'Lone Survivor,' that topped the 'New York Times' bestseller list and attracted Hollywood interest. "Every director lined up to get his story," Berg says. "I showed him a rough cut of my movie 'The Kingdom,' we had a couple beers, and then he shook my hand and said, 'You got it.' "

The son of a Korean War Marine, Berg, 51, says he has always regretted not serving. "If I could go back and do it again, I'd go for the SEALs – I don't know if I would have made it, but I certainly would have tried." As part of his preparation for the film, Berg embedded in 2009 for a month with SEAL Team 5 in northwestern Iraq – an unprecedented feat of access – since the men conducted nightly missions along the Syrian border. "I watched them operate, and when they operate, they kill people," Berg says. "These are guys in their mid-twenties who fly into hostile country and make complicated decisions we can barely comprehend – I was blown away by their self-reliance." The director became friendly with the SEAL community, inviting members to a boxing gym he owns in L.A. and to his house in Montana. "For a while I'd wake up and there'd be, like, eight SEALs passed out in different parts of my house," Berg says. "Once, in Montana, one of the guys saw a big elk, grabbed his shotgun, and killed it. I was in New York at the time, but when I came home there were 30 venison steaks completely prepped and ready to go in my freezer."

For five years, Berg worked on the script while personally visiting the families of the men who died. "The responsibility of getting this right first hit home when I visited Danny Dietz's parents' house in Colorado," Berg says. "His dad took me into Danny's untouched room – there were trophies, Hot Wheels, pictures and stuff – sat me down on the bed, and read me the military autopsy report detailing each bullet hole up to the 11th one that entered his child's head. He began crying, his tears hitting the report, and then set it down in my lap and said, 'That's who my son was and that's how hard he fought — so you get that right.' "

A full hour of the two-hour film is devoted to the SEALs' epic battle – imagine 'Saving Private Ryan's' blood-spattered Omaha Beach landing, but twice as long – a brutal sequence that also richly conveys the soldiers' devotion to each other: They suffer head-shot wounds and still manage to give each other cover and crack jokes. "I used the book as a guide and then observed SEALs battle in Iraq," Berg says. "Gunfights are like pieces of music – no one is firing and you're listening to a chipmunk, and then suddenly it becomes very hectic."

On set, Berg invited Luttrell and 10 other SEALs to help oversee the production for accuracy. "You knew these soldiers had been good friends with the guys who were dead," Berg says. "There were times when they would come up to me and say, 'This isn't right – we wouldn't do it like this.' And then we'd fix it." In addition to helping with the actors' live-fire training, Luttrell advised the film's stuntmen as they re-created the battle on a Santa Fe mountaintop. To accurately portray a deadly 40-foot cliff-side leap the SEALs survived, the stuntmen jumped off two-story boulders without any padding below. "The risk was high," second unit director Kevin Scott says, "but Pete wanted to convey the impact these SEALs felt when they landed – so our stuntmen went big, and the noise and hits in the movie are real." Luttrell appears in two scenes – lightheartedly hazing a rookie SEAL before the mission and later as one of the 16 soldiers aboard the doomed helicopter that attempted to rescue him: The camera stops on Luttrell and several real-life SEALs as the Taliban's rocket-propelled grenade enters the Chinook through its open bay door. "I was on the other side of the mountain when those guys came to help me," he says, "so getting to die on the helicopter in the movie was a very powerful moment for me."

For Luttrell, 'Lone Survivor's' release has thrown a spotlight on his still unfinished mission – to win U.S. asylum for Mohamad Gulab, the Afghan tribesman who saved the SEAL from the Taliban. Shortly after Luttrell was airlifted to safety by Green Berets in 2005, Gulab and his family became top Taliban targets. "They have a bounty on his head," Luttrell says. "He's been shot, his car's been blown up, and his house has been burned down." Gulab soon reached out to Luttrell, who arranged for the Afghan to visit him on his ranch northwest of Houston. "We walked around a lot," Luttrell says. "He's not a big TV guy." Despite not speaking the same language, the two men became inseparable. "I saw pictures of them drinking Big Gulps at the 7-11," Berg says. "They have a very clear way of communicating that doesn't involve much except a look, a hand gesture, and a smile – it's a remarkable friendship to see." Luttrell and Berg knew the film would bring renewed Taliban scrutiny upon Gulab and are currently working on obtaining U.S. visas for the Afghan and his family. "I knew we needed to get him and his family out of Afghanistan and offer asylum if he wants it," Berg says, "but Gulab is a proud fighter. His attitude is, 'I sleep with two AKs; if they want to come, they know where I am.'"

In the wake of 'Zero Dark Thirty' and 'No Easy Day,' the Special Forces top brass had initially been wary of the movie – but U.S. Special Operations Commander Admiral McRaven high-fived Berg after a screening and pledged support. The director went on to screen the film for SEALs, SWAT teams, and military around the country. "I have so much respect for the reports I see on the news now," Berg says. "When we hear that four SEALs were killed or two Rangers died or three Marines were blown up, we don't have time to process any of it because the news cycle moves so quickly – but these are all stories."