An envelope is dropped on the desk of Eliot Ness in one scene. It is assumed to be a bribe, but the amount inside is never revealed. In real life, Al Capone promised Eliot Ness that two $1,000 bills (about $46,000 - $48,000 in 2025) would be on his desk every Monday morning if he turned a blind eye to Capone's bootlegging activities. Ness refused the bribe, and in later years struggled with money. He died almost broke at the age of 54.
In real life, Al Capone actually issued a non-violence order on the Untouchables. Killing a Prohibition agent would lead to more trouble than he or his outfit could handle. Capone repeatedly tried to buy them off, but he never once attempted to kill Eliot Ness or any of his men.
Albert H. Wolff, the last survivor of the real-life Untouchables, was a consultant on this movie, and helped Kevin Costner with his portrayal of Eliot Ness.
Robert De Niro tracked down Al Capone's original tailors and had them make him some identical clothing for the movie.
Eliot Ness and his role in bringing down Al Capone had been completely forgotten at the time of his death in 1957. No Chicago newspaper carried news of his passing. His heroic reputation only began with the posthumous publication of the Untouchables book he had co-written with Oscar Fraley, and Die Unbestechlichen (1959) television series adapted from it.
Brian De Palma: [filming technique] The close-up of Malone and Ness in the blood oath scene was shot using a split focus diopter. This is a half convex glass in front of the actual lens to make one half nearsighted and the other farsighted. The result is foreground and background (here both actors) are in focus. The effect can only be achieved when there is nearly no actors nor camera movement, and is often used to create more mystery. Brian De Palma has used this in many of his movies.