To make the movie as authentic as possible, director Paul Greengrass cast a number of real-life participants in the events of September 11, 2001 to play themselves. The principal "real-life role" in the movie is Ben Sliney, the FAA's National Operations Manager, who made the decision on 9/11 to shut down all air traffic operations in the United States. Sliney had just been promoted to the National Operations Manager position, and September 11, 2001 was his first day on the job. (This explains why he receives applause from the FAA flight monitors when he walks into the control center in Herndon, VA, at the beginning of the movie.) Several officials who were with Sliney in the FAA control room on 9/11 play themselves (including Tobin Miller, Rich Sullivan, and Tony Smith). In the scenes at Newark Airport, several air traffic controllers who were in the Newark control tower on 9/11, and who witnessed the air attacks on the World Trade Center, play themselves. In the scenes at the air traffic monitoring centers in Boston, New York, and Cleveland, the air traffic monitors are all played by real-life air traffic controllers, including several who were at these locations on 9/11, and who monitored the hijacked flights. In the scenes at the Northeast Air Defense Command Center (NEADS) in Rome, NY, most of the military personnel are played by real-life military air traffic controllers, including several people (notably Major James Fox) who were at NEADS on 9/11. Also, on United Flight 93, the actors playing the pilots in the movie are real-life airline pilots, and the stewardesses are played by real airline stewardesses, some of whom actually work for United Airlines.
The actors who played the terrorist hijackers and the actors who played the passengers and crew on the flight were kept in separate hotels during filming. They also worked out in separate gyms and did not eat meals together. This was so that the director could capture the separation, fear and hostility between the two groups of antagonists and protagonists.
Families of the 40 passengers and crew members killed on United Flight 93 cooperated in the production, offering Paul Greengrass detailed background about their loved ones, down to the clothes they wore, what reading materials or music they had with them and what sort of candy they might have snacked on aboard the plane.
Rebecca Schull plays Patricia Cushing, a real life victim of the 9/11 attack as a passenger on United 93. Schull is most famous for her role on the TV show Wings (1990), created by David Angell, himself a real life victim of the 9/11 attacks as a passenger on American Airlines Flight 11.
Initial screenings ended with the closing credits line "America's war on terror had begun." This was replaced in the release version with "Dedicated to the memory of all those who lost their lives on September 11, 2001."
Almost none of the passengers in the film are referred to by their names. Their identities remain anonymous, emphasizing the group effort over any individual heroics (and also portraying the fact that strangers on an airplane would not know one another's names).
The Iraqi-born (but London-based) actor Lewis Alsamari, who plays the lead hijacker in the film, was denied a visa by US immigration authorities when he applied to visit New York City to attend the premiere, despite having already been granted asylum in the United Kingdom since the 1990s. The reason given was that he had once been a conscripted member of the Iraqi army - although this was also the grounds for his refugee status after his desertion in 1993.
There are no indications that the German passenger Christian Adams really acted the way he did in the movie. The story of the only European victim trying to stop the others from attacking the terrorists is fictitious and not based on any recordings or phone calls made in the plane.
The pins on the uniform worn by Trish Gates in the film, are actual airline pins that belonged to flight attendant Sandra Bradshaw. They were donated by her husband, Phil Bradshaw, to replicate her uniform for the film. Gates was herself a flight attendant for United Airlines shortly before participating in the film.
The ensemble cast of mainly unknown actors were each given studies of their real life United Flight 93 counterparts. Director Paul Greengrass was then able to partially improvise some of the events for the film.
Shortly after Mark Bingham (Cheyenne Jackson) gets on the plane, he strikes up a conversation with Tom Burnett (Christian Clemenson) about the rugby union football club to which he belonged. Bingham played amateur rugby with the San Francisco Fog, a Bay Area-club that encourages the participation of people of color, gay men, and women. Most of the Fog's members are gay men, as Bingham himself was.
On September 11, 2001, the aircraft on the flight was one of the four planes hijacked as part of the September 11, 2001 terrorist attacks, possibly intended to crash into and destroy the United States Capitol building in Washington, D.C. It was the only one of the four planes that did not reach its intended target, instead crashing near Shanksville, Pennsylvania, about 150 miles northwest of Washington.