Any police officer seen drinking alcohol on-screen during the film gets killed.
Malone swears that Georgie will "be begging to talk." After his gruesome "execution" of a mobster, Georgie quite literally IS begging to talk.
Al Capone's wife, Mae, was famous for being reclusive towards cameras... she would pull up her mink coat lapels to hide. She's shown doing exactly this in her only scene (at the Opera).
When Nitti tells Capone of Malone's assassination during a performance of Pagliacci, he chuckles to himself. What line is the tragic clown singing?
"Ridi, Pagliaccio, sul tuo amore infranto! Ridi del duol, che t'avvelena il cor!
"Laugh, clown! At your broken love! Laugh at the grief that poisons your heart!"
Ness' bluffing of the Judge isn't as much of a gamble as one might think. The evidence Ness provides is irrefutable. For the judge to so casually dismiss it is awfully fishy. While his name isn't in the ledger, he certainly is getting bribes somehow.
Ness saying he would have a drink if they repealed prohibition. Elliot Ness developed an extremely bad drinking problem later in life.
Malone picks Stone, the second best shot from the police academy as a recruit for their Untouchables team, as the best shot, a chronic stutterer, can barely put together a coherent sentence. Malone foresaw that this man would not have the steel nerve needed for the tasks they would face; Stone is cool as a cucumber and fires with unerring aim when he kills the mobster holding the bookkeeper hostage.
There's another possible reason for hiring Stone as well. Remember, Malone and Ness are worried about being infiltrated by Al Capone's men. Now, anyone trying to infiltrate 'the Untouchables' would almost certainly shrug off the racist abuse in order to ingratiate himself with Malone. Whereas a man willing to turn his gun on a superior officer probably doesn't have much in his closet he's afraid of revealing.