United 93 is a straightforward recreation of the flight, the air traffic control center, and Norad. Only news footage was shown of the Twin Towers. Of course, there's only so much the director could actually glean from transcripts and cell phone records, but the film is startling in its realism.
Director Paul Greengrass made the incomprehensible real. The people on Flight 93 knew that planes hit the Twin Towers and the Pentagon. They knew they were going to die and the plane was going to hit Washington. People need to remember this--not to obsess or become paranoid, but to remember. If it's too "difficult" to remember, then one day when everybody gets too complacent again, an entire city will be gone.
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"Those who do not learn from the past are condemned to repeat it"