The scene where Al Capone (Robert De Niro) pulls out a baseball bat at a dinner party and suddenly beats to death one of his men is based on a true incident which happened on May 7, 1929. Two of Capone's most feared hit men, Albert Anselmi and John Scalise, had hatched a plot to kill Capone and take over his gang. Capone got wind of it and invited all his associates to a dinner party, including Anselmi and Scalise. In the middle of the party, Capone pulled out a baseball bat and battered both men to death, then shot them both in the head. A conflicting version of the story has Tony "Joe Batters" Accardo, one of Capone's hit men, as the man who bludgeoned the traitors to death.
Brian De Palma met Bob Hoskins over a drink in Los Angeles to discuss playing Al Capone if De Palma's first choice Robert De Niro were to pass on the role. Since De Niro didn't say yes, Hoskins told De Palma he would do it if he were available. When De Niro finally took the role, De Palma sent Hoskins a thank you note, and the studio paid Hoskins, who had a "pay or play" deal, $200,000. Hoskins called De Palma and asked if there were any more movies the director didn't want him to be in.
On the AMC Network's Movies at Our House (2002), Billy Drago, who portrays the white-suited Frank Nitti, says that while they were filming scenes on the streets of Chicago, he was told about a couple of teenage street gangs getting ready for a gang fight. At the request of the Chicago Police Department, Drago, wearing his costume and armed with his prop tommy gun, went to the place where the fight was supposed to happen. The gang members were in such awe of him that they didn't fight.
Robert De Niro insisted on wearing the same style of silk underwear that Al Capone wore, even though it would never be seen on camera. The producers, knowing DeNiro's reputation as a Method actor, gave in.
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 notes would be on his desk every Monday morning if he turned a blind eye to his bootlegging activities (an enormous amount of money then). Ness refused the bribe and in later years struggled with money. He died almost broke at the age of 54.
According to Brian De Palma and Art Linson in the DVD documentary, it was Sean Connery's idea to film the "blood oath" scene between Ness and Malone in a Catholic church. Originally it was going to take place on the street (in the same scene that follows the church scene). Connery felt that a church would be the only "safe" place in Chicago where the two characters would make such a commitment to fight Capone.
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 the television series adapted from it.
In real life, Al Capone, knowing that killing a Prohibition agent would only lead to more trouble than he or his outfit could handle, actually had a non-violence order to his men, concerning the Untouchables. While Capone did repeatedly attempt to buy them off, he never once attempted to kill Ness or any of his men.
In the original script, the final gunfight had Eliot Ness and George Stone battling Capone gunmen on a stopped train. Brian De Palma conceived the gunfight on the steps in Chicago's Union Station when Paramount decided that staging the scene and finding a 1930s period train would be too expensive.
When Capone's men are trying to smuggle the book keeper (Jack Kehoe) out of town, they are going to put him on board a train to Miami. In real life, Al Capone owned a luxurious mansion in Miami. Presumably, in the film, the mob was going to have the book keeper hide in Capone's mansion.
Paramount Pictures made this film because they still held the filming rights to Eliot Ness' autobiography which they used to produce the TV series The Untouchables (1959). Originally, Paramount intended to make this, like so many other films since, as a big screen adaptation of a TV series. However, director Brian De Palma, producer Art Linson, and writer David Mamet all felt that they didn't want to remake the series so they took their own dramatic license with the story, and the true events that inspired it, in order to make what they felt would be a good big screen epic. (This according to De Palma and Linson in the DVD "making of" documentary)
When Agent George Stone is introduced, Malone founds out that his real name is Giuseppe Petri and he was born in Italy. In Italian, Giuseppe Petri can be literally translated as "Joseph Peter." "Peter" comes from the Greek "petros," which means "stone."
Fashion icon Giorgio Armani, who provided the costumes for the film, told Brian De Palma that he should cast Don Johnson as Eliot Ness. Johnson wore Armani on TV every week on Miami Vice (1984), and Armani called Johnson his 'male muse'.
Brian De Palma took the idea of the train station scene from the 1925 Russian movie Battleship Potemkin (1925) (better known as The Battleship Potemkin). The sailors who get caught in the crossfire in The Untouchables (1987) are a tribute to Potemkin.
The radio show listened to by Eliot Ness and his wife early in the movie is an actual episode of Amos and Andy. In the episode they have just bought a clunker for their new cab company from their friend The Kingfish.
The character of Oscar Wallace (Charles Martin Smith) was loosely based on Frank Wilson, the IRS agent who worked to indict Capone for income tax evasion. Wilson had been working on this project since 1928, and had next to nothing to do with Ness and the Untouchables in real-life. Wilson was not killed by Capone, though Capone reportedly placed a contract on his life which was never carried out.
Valentino Cimo, who plays Capone's bodyguard Frank Rio (The one Ness punches in the nose and shoots at the beginning of the rail station shoot out) later went on to reprise the role of Rio in the syndicated series The Untouchables (1993).
The first liquor raid was shot on LaSalle Street with period cars and extras. Ness and his men exit the Rookery Building (between Adams and Quincy) and enter the City National Bank and Trust at 208 S. LaSalle. The building in the background (with the clock) is the Chicago Board of Trade, located at LaSalle and Jackson.
For the scene in which Malone is killed, Sean Connery did not expect the squibs to be as explosive as they were. After the first take, Connery was taken to the hospital with dust and fake blood in his eyes.
The real Frank Nitti did not die in the manner and at the time depicted in the film. He took over Capone's empire when Capone was sent to prison. In 1943, Nitti and other Chicago mob members were indicted for extortion. The mob leader blamed Nitti for the indictments and told him to take responsibility for all of the charges. Fearing a lengthy prison sentence due to his claustrophobia, Nitti drunkenly wandered to a train yard and committed suicide with a gunshot to his head.