Note: United 93, directed by Paul Greengrass should not be confused with the more melodramatic TV movie Flight 93, which aired around the same time that United 93 played in the theatres.
United 93, like Flight 93 is a dramatized account of the hijacked plane (on September 11, 2001) that crashed after after passengers, having lerned of the other hijackings, fought back. None of the passengers survived, but an few people not on the plane but involved in this played themselves in the movie.
United 93's strength lies in its focus on the mundane. There is very little music, especially at the beginning, and as the passengers get on the plane, the movie shows them doing the sorts of mundane things that passengers do when they sit down in planes. The dialogue is kept to a minimum in the earliest scenes on the plane; no one tries to say cool stuff that only characters in a Hollywood movie say. In fact, of all fictional movies I've seen, this one comes the closest to feeling like a documentary, despite not being a mock documentary; no shaky cam to be found. Rather the viewer feels that they are the proverbial fly observing actual events as they unfold.
The mundane aspects might seem dull but they are actually effective. The viewer knows that no one is going to survive, so all the mundane happenings actually serve to heighten the tension. The viewer wants to warn them to get off the plane because they are behaving like real people. Actual dialogue from cell phone calls, air traffic control room conversations further creates the feeling that this is real and you are watching people heading to their deaths. You don't get to know a lot about the passengers (unlike Flight 93, their families are not depicted), but that's okay; in real life you rarely learn much about your fellow passengers. Also, in many ways their backgrounds matter very little; it's what they do when presented with a hopeless situation that makes them intersting, not their back story.
They are practically no Hollywood touches., no clich to undermine the sense of reality; Paul Greengrass has created a near-prefect illusion, an as exact as we're likely going to get account of what happened on that flight and at places that should have been set up to deal with such a crisis. The passengers, while heroic, aren't portryed as martyrs or larger than life people but rather scared people who have figured out they're in a pretty hopless situation and are trying one last ditch attempt stop the highjackers because frankly the alternative is to simply do nothing as the plane hits its target. s