A serious man with a serious subject: 9/11

Paul Greengrass, director of ''United 93," paced back and forth. He leaned against a floor-to-ceiling picture window facing Avery Street. He touched the glass, as if pulling an answer from the blue sky, and Boston beyond.

''Some people out there may wonder, hearing about the film, 'Have they turned it into entertainment?' " mused Greengrass, whose new film is scrupulously based on the events of Sept. 11 -- specifically, the doomed United Airlines Flight 93 that crashed at 10:10 a.m. in Shanksville, Pa., killing the plane's four hijackers and all 40 passengers and crew.

''As to whether an audience comes to see it, there will be people who will not want to, and that means the events are too painful," Greengrass said. ''I understand and respect that."

Wearing a black pullover, jeans, and sneakers, the 50-year-old Greengrass looked tired. He took off his gold wire-rimmed glasses, rubbed the bridge of his nose, and ran a hand through his mane of graying hair. He had only finished post-production two weeks ago.

Despite the fatigue, and criticism that the almost five years since 9/11 still do not provide enough breathing space for such a troubling film, his resolve is strong. Greengrass is a serious man, haunted by serious subjects.

''What do we do in the post9/11 world? How do we respond to the challenge of 9/11? What does 9/11 mean to us today?" he asked. ''All across the world, there's this passionate, engaged, divided conversation taking place, and I think that it's time for filmmakers to join that conversation."

To seek answers, Greengrass traveled to what he called the ''common ground" of those two hours that September morning.

''United 93," which opens today, brings the viewer, step by harrowing step, through the day's events. We get no teary breakfast goodbyes or back stories -- just hijackers bowing at their morning prayers before entering the Newark International Airport, anonymous as their fellow passengers. As the crisis unfolds, the film intercuts among the civilian and military authorities grappling to make sense of the chaos from the ground and, in the air, the unthinkable -- and partially unknowable -- final minutes of Flight 93.

''[The filmmaking team] gathered together in an attempt to draw this story together and distill it and try and find some meaning in it," Greengrass said. ''I think that is a reasonable and responsible objective."

''United 93" is a return for this TV journalist-turned-filmmaker to the hyperrealist territory of 2002's ''Bloody Sunday," which re-created the 1972 massacre of Irish civil rights marchers by British troops. Greengrass surprised many by following up that much-admired film with ''The Bourne Supremacy," the 2004 sequel to the massively successful Matt Damon thriller ''The Bourne Identity."

Greengrass differentiates between a ''franchise popcorn thrilling adventure ride" such as ''The Bourne Supremacy" and, he said, films such as ''United 93": ''cinema as a piece of cultural experience." He trusts audiences can discriminate, too. ''I think that a million times over they will know the difference with this, because of this subject matter."

To make ''United 93" a cultural experience required painstaking footwork -- combing through public records such as the 9/11 Commission Report and newly declassified documents and using material from dozens of face-to-face interviews. ''Ultimately, you piece it together," he said. ''We did a pretty good job of it and of reaching out to people who were really there."

Part of that outreach involved talking to the families of the dead, a job that fell to associate producer Kate Solomon, who crisscrossed the country to conduct some 140 interviews. Most families cooperated, Solomon said, and provided details about how their loved ones wore their hair, what clothes they had on, what newspaper they read, and how they took their coffee. ''They were reassembling them," Solomon said. ''It was very important to them that they were remembered."

The sets were, according to Greengrass, ''actual facsimiles" of air traffic control centers in Boston (where the hijacked American Airlines Flights 11 and 175 originated) and New York, the control tower at Newark, the Federal Aviation Administration's command center in Herndon, Va., and the military's Northeast Air Defense Sector operations center in upstate New York.

For the plane, Greengrass shot inside an out-of-service Boeing 757 reconstructed on London's Pinewood Studios soundstage. Set designers mounted pull-apart sections of the fuselage on motion gimbals; this re-created the lurching motion of the plummeting plane. The jittery, hand-held camera -- sometimes affixed to a bungee cord -- and telephoto lens added to the sense of claustrophobia and chaos. ''The shallow depth of field makes the scene incredibly dense," the director explained. ''You were struggling for control of the camera . . . the way the passengers and terrorists were struggling for control of the plane."

Also contributing to the verisimilitude was Greengrass's risky choice to use nonprofessional actors: actual military commanders, flight attendants, and civilian aviation officials, many of whom had never acted before. In a bizarre if cathartic twist, several re-created their own real-life 9/11 roles, including local air traffic controllers Tommy Roberts and Colin Scoggins, who played themselves. The hijackers were played by four British men, one of Moroccan descent, two of Egyptian, and one of Iraqi.

Portraying Flight 93 captain Jason M. Dahl was actual United Airlines pilot J.J. Johnson, whose sole acting experience was his eighth-grade play.

''I actually flew that plane for seven years. I flew that route from Newark to San Francisco," Johnson said. ''[Greengrass] wanted the piloting part of it to be real. . . . It was an added bonus that I was the same size and the same height [as Dahl]." Johnson did his own stunts and improvised many of his lines. At the Tribeca Film Festival this week, he was set to meet Dahl's wife, Sandy, for the first time.

Ultimately, Greengrass wanted his film to have a thriller's breathless quality but not seem exploitative or contrived. And he wanted to do justice to the dead and survivors. ''These people were doing their best. They were overwhelmed, but they had the courage to keep on going."

One scene, set at the Northeast Air Defense Sector, was cast with military personnel reenacting the events of that September morning. ''We were filming that first long take. It was just like a military exercise," Greengrass said. The shot included a woman named Shauna Fox, a communications expert, whose job was to process information and pass it along. ''I was watching her field this information," he remembered -- the first plane, the second, the CNN live footage.

''I saw her and I saw the immense, immense shock in her face. The hair went up on the back of my neck. I thought, she's not acting it, she's reliving it," Greengrass said, his gaze still on Boston, or, perhaps, beyond, to New York, Washington, or some field in the Pennsylvania countryside.

Ethan Gilsdorf can be reached at ethan@ethangilsdorf.com.