Bravery and heroism are notions ascribed after an action. Provided someone’s heart is in the right place, something heroic comes not from intent of earning respect but the instinct of reaction.

Nearly five years removed from 9/11, it’s admittedly sometimes hard to remember that split-second decisions were all anyone had on that day of unimaginably swift evil. Lengthy trials, commission reports, changes in air travel, tapes and transcripts, partisan politicking, intelligence scrutiny, the war on terror — all news nuggets we regularly associate with that day.

The exact truth of what happened on United Flight 93 — the only terrorist-controlled airplane that didn’t hit its intended target and one on which passengers rose up against their captors before a fatal crash with no survivors — never can be fully known.

But United 93, a film chronicling the plane’s real-time flight, plays out as a wholly respectful re-creation of these passengers’ primal drive to reclaim control of their fate — their need to act not even necessarily in terms of survival, but at least trying and, if they must, dying on their own terms.

When the plane initially turns around, these people have every reason to believe they’re heading to an airport for a ransom standoff. To a point, they presume the pilots are still operating the plane. When those circumstances change, writer-director Paul Greengrass forces us to not so much relive the events of the day, but examine something of ourselves in what we learn about these passengers. Could we, on the verge of death in a situation for which we had no mental template, find the internal strength to react in the same way?

Greengrass mastered a similar microcosmic realism in Bloody Sunday and here interviewed hundreds before writing his script, split between United 93 and responsive efforts from the Northeast Air Defense Sector (NEADS), Federal Aviation Administration and air-traffic controllers.

In spending so much time on the ground, Greengrass asks the flip side of the previous question: Tasked to respond to such an unanticipated tragedy, could we, as officials, react with clear heads? It might be easy to get outraged at some of these agencies’ delays, but that would be purely hindsight anger while watching an in-the-moment movie that points no blame. Greengrass allows the audience to draw its own conclusions of responsibility from the timeline provided.

Many of these administrative officials play themselves, most memorably Ben Sliney as the FAA’s National Operations Manager — running numbers and being ribbed about budgetary concerns before a call to the floor where he incredulously reacts to reports of a hijacking.

At NEADS, planned simulations are junked (“This is real world,” is the repeated military speak) and chain-of-command lines initiated to determine if, and how, to engage these planes. Meanwhile, without any strain of movie manufacturing, taxing tension is wrung from air-traffic controllers hearing snippets of terrorist cockpit chatter, pulling tapes and discovering ghastly new information. (Greengrass and composer John Powell know just how to use, or not use, a very affecting score.)

Some of those actors portraying passengers on United 93 have mildly recognizable faces from commercial or feature work (namely Christian Clemenson, Denny Dillon, David Rasche and John Rothman). But their blend-in ability holds up the mirror reflection; this could have been anyone.

It’s not unreasonable to want to know more about these people, but, although Greengrass spoke with their families, it’s not what he’s attempting to say. (Those seeking more background from a filmic perspective should view Discovery Channel’s fine The Flight That Fought Back.)

The passengers’ retaliation comprises the film’s final minutes and it’s almost eloquent in its fury — no slow motion, rousing music or showboating effects, just frenzied activity and an indelible final image. It’s here that United 93 swells with immeasurable heart and hope where it could have been empty or crassly exploitative, and its exhausting, emotionally draining greatness comes from an unadorned story of humanity’s personal resolve, not politics or patriotism, in one finite moment.