United 93 doesn't have a "plot" or "characters" in the conventional sense. Most of the characters are never named. Only one that I noticed - Ben Sliney - is ever identified by first and last name. Two we recognize - Mark Bingham and Todd Beamer - but most are simply there. They appear onscreen as people appear to us in life, without introduction or fanfare.
Many of the surviving characters are played by their real-life counterparts. Look at the cast list and the first thing you will notice, after the absence of "superstars", is the number of roles identified as "Himself/Herself". Many others are known only by their role titles, "Boston 5", "Herndon 1", "Cleveland Supervisor".
It is a cast of walk-ons - again, just as in life. And yet paradoxically, it's a movie about ordinary people who become actors: not "actors" in the sense of performers, but actors in the original sense - people who take action.
The movie does not try to "explain" United 93. It does not bother to fill you in on what everybody knows, because you already know it. It's very much like the classic horror device, where you (the viewer) know the danger awaiting the characters, but the characters themselves are blissfully ignorant. In other words, it is simply excruciating.
I like relationship films. I cry in movies. I cried in "United 93" because it is a movie full of ending relationships. To say that there is "no happy ending" doesn't begin to tell it. There is not one tragedy but many: the sundering of romances we have not seen develop, the bereavement of families we've never met, the goodbyes to people we will never know. There is no redeeming moment at the end; no "boy gets girl"; no tender embrace amid the smouldering rubble, pregnant with the hope of life renewed. No. There is only the spinning ground and the silent blackness.
Did I give away the ending? But you already knew how it would end.