At this early stage, the Stone film - which takes place firmly on the ground - looks to be the most conventional 9/11 movie yet: a rousing hymn to what its trailer describes as "courage and survival". United 93, however, takes an entirely different tack: this remarkable experience swoops in, appropriately enough, out of the clear blue sky. Though Greengrass's film won't appear entirely out of place in a multiplex still showing disaster movies (Poseidon) and works where the audience will mostly be aware of the ending going in (The Da Vinci Code), it might seem curious that it should be released at the height of summer - particularly a summer with a distraction as big as the football World Cup on - rather than during awards season.
But this is Greengrass's trump card: just when you turn up at the cinema thinking you already know what a United 93 movie is going to be, the writer-director makes surprise aesthetic choices, hijacks expectations; again, appropriately enough, its emotion comes out of nowhere. For its first half, United 93 offers an unexpectedly compelling lesson in how air traffic control works, and then what happens when it doesn't: all those blips moving unpredictably across radar screens, the frantic tin-pushing of men in short sleeves. The second half is then reserved for a real-time recreation of what happened in the air aboard Flight 93, where the passengers reportedly stormed the cockpit and fought back against the hijackers - not because they wanted to die, but because they wanted to live. All this is set out in the (itself unusual) drama-documentary style Greengrass first set out in Bloody Sunday and then smuggled into the Matt Damon blockbuster The Bourne Supremacy.
After the Bourne film's success, there was a danger Greengrass might have been absorbed into the system as just another action specialist, but here he stages a series of tense set-pieces in a manner rarely seen in major American releases: low-key, with no big names or recognisable faces among the cast, and a superior eye for the telling detail. In doing so, he avoids slanting the film in any particular direction. United 93 is not meant as propaganda for the War on Terror, nor can it be understood as an anti-War tract, although it sheds tangential light on George W. Bush's subsequent reaction to these events: precisely that of a man who's just had someone attempt to fly a commercial jetliner into his front room.
The film's methodology is, I think, closer to American procedural television: shows like Law & Order or CSI, which work through case studies with an aim to reassuring us that even the most aberrant and abhorrent events can be rationalised and understood in their own way. (Either that, or it's Final Destination for grown-ups, allowing us the cathartic, if guilty, pleasure of undergoing a lethal descent and emerging from the multiplex unscathed.) What Greengrass's film unquestionably is, though, is true to its particular, troubled moment in history. United 93 deals in real drama and real tragedy, shooting down the fevered conspiracy theory with a few sobering lines of postscript, and achieving the near-miraculous feat of bringing casualty statistics - both terrorist and victim - back to life as human beings: scared, misguided, vulnerable and heroic.
United 93 screens on ITV1 tonight at 11pm.