Adipose Rex: Capone is an obese kingpin. Robert DeNiro gained 15 pounds for the role, which is not much by his standards. He wanted to double that figure, but the schedule was tight, so he had to wear pads and pillows under his clothes.
Baby Carriage: During the fight in the station, a runaway baby carriage presents Ness with the dilemma of trying to save the baby or dealing with a gun-wielding mobster. He chooses the mobster, trusting that one of his colleagues will save the baby. It's the right choice, as the colleague stops it with his legs. While still shooting down some mobsters. And the baby is fine in the end.
[Ness refuses a bribe from Capone's lackey] Alderman: You're making a mistake. Ness: Yeah I know, well I've made them before I'm beginning to enjoy them. Alderman: You fellows are untouchable, is that the thing? No one can get to you? Hey, everyone can be gotten to. Ness: You tell Capone, that I'll see him in hell.
Batman Gambit: Ness bluffs the judge overseeing Capone's trial to switch juries (as Capone's had been bribed), by telling the judge his name was in the ledger listing all of Capone's payouts. The DA notes that they didn't find the judge's name in that ledger...
Batter Up: Al Capone famously used a baseball bat to savagely murder one of his subordinates.
Don't insult Stone's heritage, or he'll shoot you. Malone used this as a Secret Test of Character when recruiting his team.
At the beginning of the shootout at the bridge, Oscar Wallace is cringing, with a "Oh dear God please get me out of here" look on his face. Then Stone gets shot. Cue Roaring Rampage of Revenge.
Eliot Ness flips and throws Frank Nitti off a building when he gloats about killing Jim Malone.
Bittersweet Ending: Capone is behind bars, his deadliest enforcer Nitti gone and his criminal empire shattered. But two of the Untouchables are dead, and Ness had to violate a lot of his personal code of honor - including his tossing Nitti off a roof in revenge for Malone's death - to get Capone.
Black and White Morality: Capone orders the deaths of children and beats men to death with baseball bats. Ness is all-American Hero who loves nothing more than his family and getting notes from his wife in his lunch. As the film goes on, it turns into Black and Gray Morality as Ness uses more questionable means to bring down Capone.
Blood Brothers: Malone takes Ness to a church, where they talk about a Blood Oath and Malone tells Ness they are now bound by it.
Bond One-Liner: A classic Non-Bond example. Happens after Ness has thrown Frank Nitti off the roof of the courthouse and he lands on the roof of a Model A:
The mobster with the bowtie. He is seen several times, particularly when he lures Malone out to be shot by Nitti and is later the one who takes the bookkeeper hostage on the train station steps and gets shot by Stone.
The mobster Ness punches in the face when he tries to stop Ness from confronting Capone. Unfortunately this means he remembers Ness, which sets off the shootout when he sees Ness with the Baby Carriage at the railway station.
Fake Kill Scare: Done with a twist — Malone scares the crap out of a captured smuggler by letting him see Malone demand that his accomplice reveal information, threaten to blow his head off if he won't talk, and then literally blow the accomplice's brains out. What the surviving captive (who, terrified, tells all he knows) didn't know was that the accomplice had already been fatally shot in the gunfight just before: Malone was "interrogating" a corpse.
Malone What are you prepared to do? Ness Everything within the law. Malone And then what are you prepared to do?
Going by the Matchbook: Frank Nitti has the address of Jim Malone written on a book of matches. Unfortunately for him he does not dispose of it after killing Malone, so when Eliot Ness searches him later it leads to their final shootout.
Hidden in Plain Sight: The bookkeeper and his mobster bodyguards walk right past Elliot Ness at the train station, as all they see is a man helping his wife pull their baby carriage up the stairs. Unfortunately the last mobster to enter the station has a bandaged nose from Ness punching him in the face, and remembers the man who gave it to him.
When they discover the jury has been bribed, they switch Capone's jury with the jury next door. Something like this really happened, but not the way it's depicted in the film. Of particular note, in real life the jury was switched much earlier in the trial - the pool of jurors both sides could select or veto was switched; switching it when they did in the film, even if it had been allowed, would have meant that the new jury was handicapped by having missed the presentation of key evidence.
Capone's lawyer attempting to enter a plea without his client's consent is not based on reality, and, in real life, is a good way to have a mistrial, an overturned conviction, and disbarment for the attorney.
Idiot Ball: Ness and Co are in a corrupt town, they know they can't trust most of the police and what do they do with their key witness? They put him in an elevator with just one Untouchable (Oscar the accountant) and don't bother to clear out the elevator of anyone they don't trust. They were really holding onto it that day.
Infant Immortality: Averted and subverted. Ness' own young children, prominently featured throughout, survive without a scratch. On the other hand, one of the first scenes in the movie shows a gangster walking out of a bar, and a little girl running after him, carrying him the suitcase he left behind. Guess what's in the suitcase...
Justice by Other Legal Means: Getting Capone on tax evasion, of all things. Even Ness lampshades it in the movie when his accountant sidekick keeps pointing it out. (Though a standard legal tactic today, convicting criminals on tax evasion was a radically new idea at the time, and though it had been used successfully as far back as 1927, no one had ever tried it on so large a scale, or against so prominent a criminal.)
Naturalized Name: George Stone's birth name is Giuseppe Petri, a close-enough translation ("Petri" is close to pietre, Italian for "stones", but Giuseppe's English equivalent is traditionally Joseph, not George).
Reliably Unreliable Guns: The tommy gun of one gangster jams during a fight. (Truth in Television: a problem real tommy guns were frequently subject to, which is one of the many reasons it was never as popular as gangland movies would have you believe.) To the gangster's credit he tries repeatedly to clear the jam, but it gives the mousy accountant among the Untouchables time to get close enough to KO him with the butt of his shotgun.
Team Shot: See the image up there. Also a in-universe example, the four pose for a family photo in a restaurant.
Title Drop: "You fellows are untouchable, is that the thing? No one can get to you? Hey, everyone can be gotten to."
Too Dumb to Live: No, Mr. Nitti, I don't think it's a particularly good idea to taunt Ness about his partner you murdered when both of you are standing on top of a tall building.
Took a Level in Badass: Oscar, the federal accountant assigned to Ness's team. More an office worker than a field agent, he takes to wielding a shotgun pretty quickly and gets a few Moments of Asskickery during the Canadian border raid.
Very Loosely Based on a True Story: Ness and his Untouchables really existed. They really did get Capone on tax evasion. The baseball bat scene and the jury switch are based on real events. On the other hand, quite a lot of the story is just plain made up. The depiction of Ness owes more to dramatic effect than historical accuracy, and the rest of the Untouchables depicted in the film are entirely fictional. In real life there were 11 "Untouchables" not counting Ness, and they all lived to see the end of Prohibition. So did Frank Nitti, who took the reins of the Chicago Outfit after Capone went to prison, successfully diversified the Outfit's interests to keep it going after Prohibition ended, and eventually died by his own hand in 1943. The tax evasion case was based on information collected by Frank J. Wilson, from the Treasury Department's Bureau of Internal Revenue; the credit for bringing down Capone belongs to him, not Elliot Ness. Al Capone and Eliot Ness never met face-to-face.
RCMP Captain: I do not approve of your methods. Eliot Ness: Yeah, well... You're not from Chicago.
What You Are in the Dark: Ness is sorely tempted to shoot Nitti during the rooftop chase, when it's just the two of them there. He decides against it and arrests Nitti instead. But then Nitti just had to go taunting about how Malone died by his hand, and how he'll still beat the rap...
Villainous Breakdown: Where to begin: as soon as the Judge decides to switch the Juries, Capone freaks out at his lawyer. As soon as his lawyer switches pleas, Capone punches him. When Ness gloats to his face, he repeats the phrase "You're nothing but a lot of talk and a badge." to the point where the more he says it, the more he realizes his life of crime is over.