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The Department of Defence gave the producers a platoon of Army Rangers who did the fast rope scenes.
Some of the radio chatter in the film was taken from actual radio transmissions made during the battle.
The Black Hawk going down, spiralling as it crash-lands, was achieved largely through real, skillful flying of the helicopter, with some CGI augmentation. The minute it hits the ground, however, the rotors are computer-generated.
Nelson (Ewen Bremner) says to Twombly "Just don't fire that thing so close to my head, I can barely hear as it is." Bremner actually partially lost his hearing because of all the gunfire. He eventually recovered from it.
On the last day of their week-long Army Ranger orientation at Fort Benning, the actors who played the Rangers received a letter that had been anonymously slipped under their door. The letter thanked them for all their hard work, and asked them to "tell our story true", signed with the names of the Rangers who died in the Mogadishu firefight.
The set was constantly bothered by stray dogs running into shots. Ridley Scott kept them in because he liked the authentic feel of their presence. Eight dogs were adopted by various members of the production and were eventually brought back to the US with them.
Ridley Scott: [ceiling fan] There is a large ceiling fan in the scene where Garrison interviews Mohammed Atto.