Sometimes it’s necessary to think inside the box.

That was the strident belief of Rodrigo Cortés, the Spanish director behind “Buried,” the most deliriously claustrophobic film imaginable – the entire movie is set inside a wooden coffin beneath the sands of the Iraqi desert, and the only person ever shown on screen is Paul Conroy(Ryan Reynolds), the desperate man trapped inside that casket.

The unsettling film, which opens Sept. 24 in limited release and then goes wide on Oct. 8, uses an intricate system of sliding panels that allowed Cortés and cinematographer Eduard Grau a wide array of unexpected vantage points and dynamic storytelling opportunities. Conroy’s cellphone connects him to the world above and to the unfolding mystery of predicament while his Zippo lighter’s flickering flame illuminates his face and his fear. Still, despite all that, the film is 94 tense minutes spent inside of a box, and Cortés concedes that there were plenty of skeptics as the project got underway.

“From the very beginning, I received every kind of, um, let’s call them kind suggestions to take the camera beyond the coffin,” Cortés said. “I was told it would bring some oxygen to the audience if we were to show the surface or to cut out to the other side of the [phone] line, for instance, or if we showed the other characters, like the leader of the hostage-taking group or his wife or the federal authorities. There was talk of doing flashbacks. All of this, I thought, was the perfect way to spoil everything and ruin the film.”

Behind the scenes with director Rodrigo Cortés and Reynolds on the set of "Buried." (Lionsgate)

The keep-a-lid-on-it aesthetic choice appears to have been the right one. In January, the movie earned some especially enthusiastic reviews at the Sundance Film Festival — Rob Nelson ofVariety, for instance, called it “an ingenious exercise in sustained tension that would make Alfred Hitchcock turn over in his grave” – and sparked a bidding war that ended with Lionsgateacquiring the North American rights for a reported $4 million. “We were absolutely determined to bring it home,” said Jason Constantine, the Lionsgate president of acquisitions and co-productions at the time.

The biggest challenge the film may face may be the anxiety of moviegoers – how many people are willing to watch a film that is an extended panic attack? As Cortés put it, it’s “something you feel more than you watch; it’s extremely visceral.”

The movie starts with several long minutes of complete darkness and the labored breathing of an injured man. Then, as he regains consciousness, the sounds switch to hyperventilating terror, clawing and whimpering. Reynolds said the performance was a hard one because “it was so vulnerable and raw” and required him to “do things that as a man we don’t usually show” when it comes to pure, sputtering fear.

Reynolds plays a truck driver confined in an underground coffin in Iraq. (Lionsgate)

The movie benefits, certainly, from the surging profile of Reynolds, who starred with Sandra Bullock in last year’s romantic comedy hit “The Proposal” and also has the title role in “Green Lantern,” the 2011 superhero film that Warner Bros. hopes will yield a major franchise.

Reynolds said he was instantly intrigued byChris Sparling’s script for “Buried” but became passionate about  the project after meeting Cortés.  After a 40-minute lunch, he agreed on the spot to make the film. “Buried” is just the second feature film for the 36-year-old Cortés, and he’s an unknown to American audiences, but Reynolds gushed about the director and said he’s eager to work with him again even though “we have very different tastes in material, quite honestly.”

Reynolds has worked with proven veteran directors such as Martin Campbell andJoe Carnahan, but he said he took more away from his whirlwind collaboration with Cortés than from any other filmmaker. “I call him an architect, not a director,” Reynolds said. “This is a guy that can make any movie of any size, and believe me, he will be doing alright after this.”

The film was shot over 17 days in Barcelona, and Reynolds still shuddered when he recounted the experience, which had him in a box, often in the dark and wailing for help. He charred his fingers black while holding the cigarette lighter in awkward positions and gobbled mouthfuls of sand during a scene where part of the casket buckles. At one point, Reynolds had to share the tight space with a snake that, in the story, slithered into the coffin through a crack.

Paul Conroy (Reynolds) intently watches his cellphone -- his lifeline. (Lionsgate)

“I spent the days in such a heightened state of anxiety and panic that I don’t remember a lot of it, and it’s hard to watch it now,” Reynolds said. “I’ve seen it twice, and it’s hard to see. It feels a lot like a fever dream. There was points where I’m in there screaming that I’m running out of air, that I’m running out of time, and the crew kept ripping the lid off the coffin. They thought I was really in distress. It happened like three different times. It reached such a fever pitch of panic, they thought it was real. I told them, ‘If I need help, I’ll yell, ‘Cut!’ It was harrowing for everyone. I didn’t sleep well, and it really stuck with me. You’ll never complain on any movie set ever again after something like that.”

Cortés said his compass point for the film was pure Hitchcock – the real-time suspense of “Rope,” the confined quarters of “Lifeboat” and the paralyzed point of view in “Rear Window.” Cortés acknowledged the movie had “topics of the moment” with the Iraqi setting, terrorism backdrop and the commentary on both corporate cruelty and government bureaucracy, but he said the greater goal of “Buried” was always to dig deep into Hitchcock’s old turf, a place of tension and intense craft challenges.

“I always saw the film as something very big, something Hitchcockian,” Cortés said with a chuckle. “When they asked me if it would be strained” by the setting limitations, “I said, ‘No, it will be Indiana Jones in a box.’ ”

Reynolds wasn’t sure about the “Raiders of the Lost Ark” allusion, but he agreed that “Buried” had unearthed something special.

“There’s a freedom in the restriction — it becomes a big movie because of those restrictions,” Reynolds said. “You see the size of the story because of that limited space. The limited space magnifies everything. … People say, ‘Weren’t you worried to do a film this risky?’ And I say, ‘No, why would I be worried?’ If it didn’t work, we all get a big pat on the back for trying something different. The main thing was surviving the shoot.”