Of the various ways to assess the understated excellence of United 93, a comment by one of the cast seems to me particularly on-target. Describing writer-director Paul Greengrass, Susan Blommaert, who plays a passenger on the doomed 9/11 flight, said, “I feel like Paul is anti-sensationalist and an anti-sentimentalist.”

While anti-sensationalism and anti-sentimentality are negative virtues, they are arguably the most important ones any film on this trickiest of subjects could exhibit. With material so emotionally charged, few filmmakers would be able to resist the temptation to hype the terrorists’ “evil” or the passengers’ “heroism,” even in ways that would strike most moviegoers as forgivable. Yet this is precisely what Greengrass has done: Create a film that’s superbly restrained in its vision of searing horror.

As the first major-studio film about the events of 9/11, United 93 obviously had various delicate challenges to negotiate. One, of course, involved timing. Seeing the film, you can’t help but realize that if it had come out in 2003, it would have inevitably seemed exploitative, no matter how much restraint it displayed. By the same token, five years from now it might well seem irrelevant. But five years after the tragedy, it arrives at a time when the kind of reflection it calls for is both appropriate and bearable.

Though Hollywood’s slow tiptoeing toward the subject may be partly understandable, it’s also noteworthy that United 93 is very much an individual filmmaker’s vision, not the product of any corporate boardroom or the standard studio process of “development.” Greengrass is a Brit with only two previous widely seen films to his credit: the politically charged Northern Ireland drama Bloody Sunday, and the hugely successful espionage thriller The Bourne Supremacy. Reportedly, he began thinking of a movie about Flight 93 shortly after 9/11, but it was only the power given him by Bourne, plus the collapse of a subsequent big-budget project, that propelled him into making United 93, which he did on a much smaller budget ($17 million) but with much greater autonomy than is customary in the realm of studio-backed productions.

Though some are likely to describe the film as “documentary-like,” that’s a slight misnomer that results from Greengrass’ decision to avoid two standard tropes of Hollywood movies. First, the film doesn’t single out a handful of characters and create mini-dramas around them, thereby increasing our identification with certain figures in the story. Rather, it depicts the people on Flight 93 as a collective, with no individual treated as more significant or central than any other.

Second, the film wasn’t scripted in the usual sense. Greengrass did extensive casting calls in the United States, looking for unknowns who were adept not at reading lines but at improvising. Once he found the actors who would play the passengers and crew of Flight 93, he had them housed collectively in London–the airliner interiors were shot at Pinewood Studios–and developed the film’s scenes with them through improvisation in rehearsal. (The actors who play the hijackers were cast in Britain and kept apart from the others until late in the process.)

United 93 begins before dawn with the four hijackers saying their prayers and preparing for their mission. (Here their words are not translated by subtitles; later they are–sometimes. The film would have done better to be consistent on this point.) Once the scene shifts to Newark Airport, the story unfolds in real time, a device that adds its own element of tension; like the confinement of the airplane, it insinuates an entrapment that we share with the passengers.

Though the narrative has a compelling concentration and simplicity, its path is not single but double. Up until the final 15 minutes of the drama, Greengrass cuts back and forth between the events on Flight 93 and several air-traffic control centers, both civilian and military. This is how we experience the overall unfolding of the 100 or so crucial minutes on the morning of Sept. 11. There are no cutaways to the other hijacked airplanes or the president or the streets of Manhattan or scrambling news crews. We witness the attacks solely from air-traffic control towers.

This technique has its own fascinations. Greengrass induced a number of the actual air-traffic controllers and officials to play themselves, and though (especially early in the film) it adds an occasional moment of stiffness, as the story rolls on the aura of engaged professionalism becomes palpable in a way that transcends any variety of contrived realism. In addition, it’s strange bordering on shocking to see how clueless these pros were through much of the tragedy. Planes fall off their screens and they have no idea where they are until they look up and see a flaming hole in the side of the World Trade Center, or glance out the window and observe the second plane flying into the other tower.

On board Flight 93, meanwhile, people order breakfast and read their newspapers and endure a delay at takeoff: mundane events that don’t need to be heightened since our knowledge of what’s to come provides plenty of dramatic torque. It’s about an hour into the film when the plane finally takes off, a sad irony given that the pilots will learn of the attacks on the World Trade Center but not be alert enough to bar their cabin door.

Greengrass based his account on reams of documentary evidence, including the report of the 9/11 Commission. More crucially, he and the film’s crew and cast contacted the families of Flight 93’s passengers and crew, convinced most of them of the worthiness of their project, and enlisted their help in fleshing out the mostly improvised script with lots of personal details. Yet as much as this provided a patina of factual minutiae–as well as cementing an emotional bond between many actors and the characters they play–Greengrass has correctly emphasized that the film is still a work of drama and invention, not a stab at unadorned accuracy.

We don’t know, after all, what really happened on Flight 93, apart from the various details from recorded cockpit conversations and telephone calls from the passengers. That leaves plenty of room for interpretation and embellishment of a sort that’s intriguingly evident, for example, in the portrayal of the lead hijacker.

Ziad Jarrah (vividly embodied by Khalid Abdalla) is a tall, balding, bespectacled young guy who looks like he could be a medical assistant or librarian. No doubt he was appointed the mission’s leader because he was educated and devout. Yet from the first he seems nervous and uncertain. One of the film’s indelible images is his apprehensive glance over his seat at the co-conspirators seated behind him in first class. For a while, it looks like he might be losing his nerve. His cohorts glare his way with mounting concern and hiss at him to start the operation.

Did it really happen like this? Who knows? The point is not just that the portrayal ratchets up the film’s suspense in a way that’s entirely credible, but that the ultimate effect is to humanize a character who could easily have been reduced to a stark caricature.

The technique works on the other side too. Once the hijacking has been brutally announced (via the stabbing of a passenger and, soon, the off-screen murders of the two pilots and a flight attendant), there’s a certain amount of chaos and panic. Yet when some of the passengers decide to attack the hijackers, it’s because they’ve grasped enough about the situation to make a rational decision. What we witness isn’t “courage” in some sort of strutting, patriotic sense but a cool, determined, very human effort at self-preservation. Likewise, when other passengers call their loved ones to say goodbye, there are tears but no hysteria. At the time, some relatives remarked on the passengers’ calmness under the circumstances; that’s what we see here.

Style-wise, Greengrass practices a kind of naturalism (hand-held cameras, quick cutting, low-key lighting) that’s become something of a cliché in the thriller genre (think of Tony Scott’s rancid oeuvre). Yet in his handling, this array of eye-grabbing techniques exhibits a very unusual combination of freshness, control and understatement that completely lifts it above the ordinary. You might credit such distinctiveness to the director’s skill or good taste. I would only add that Greengrass’ sensitivity and intelligence is matched by an unpretentious sincerity that seems to account for the careful, subdued integrity of his work.

Though it may sound stomach-churning or depressing, United 93 is, instead, sobering and thoughtful. Indeed, it’s hard to imagine a more responsible or artistically impressive film on this subject. Of course, it only shows us a small slice of the 9/11 catastrophe. We are given no sense of what led to these events, nor are we told, for example, that their consequences included a disastrous U.S. invasion of a country that had nothing to do with the attacks while the terror masterminds got away scot-free. But there’s only so much one movie can do.