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. That explains the 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. 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. 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 flight attendants are played by real-life flight attendants, some of whom work for United Airlines.
The actors who played the hijackers and the actors who played the passengers and crew were kept in separate hotels during filming. They also worked out in separate gyms and did not eat meals together. The director wanted to capture the separation, fear, and hostility between the two groups.
Families of the 40 passengers and crew members killed on United Flight 93 cooperated in the production, offering director Paul Greengrass detailed background about their loved ones, down to the clothes they wore, the reading materials or music they had, their personal characteristics/mannerisms, and the snacks they might have brought aboard.
United 93's flight crew is portrayed by real pilots and flight attendants, some of whom work for United Airlines.
US Immigration officials denied Lewis Alsamari, the Iraqi-born, London-based actor who plays the lead hijacker, a visa to visit New York City to attend the premiere, because he was once a conscripted member of the Iraqi army. He was granted asylum in the United Kingdom as a refugee after he deserted the Iraqi army in 1993.