Nucky gets more and more rude to Harlan the Ritz shoe-shiner after he begins working directly for him. This behavior highlights how Nucky is Nice to the Waiter but a Bad Boss. He's friendly to people who mean nothing to him, but treats those under his power like tools.
Nucky is a very permissive parental figure, as evidenced by his reaction to Teddy's pyromania. He gives him a little talking-to, then rewards him and sends him on his way. We later see how abusive his father was, making his own behavior easy to understand.
When Chalky White is moved to Dunn Purnsley's cell in jail, the rest of the inmates stand up and tower around him in a way that seems threatening. The moment is presented as if Chalky has lost his privileged status and is now in physical danger from the masses. The air of menace continues until we learn that all of the inmates except Purnsley are actually at Chalky's beck and call. When they stood up on his arrival, it was actually out of respect.
The way Angela's last appearance mirrors her last painting.
Richard is from Wisconsin, a state with a long history of local liberalism. It shouldn't be a surprise then that he knows the difference between a socialist and a communist, even though he himself has not much interest in politics.
Gyp is in a lot of ways an Evil Counterpart of Chalky (this is noted explicitly in "Two Impostors" wherein Gyp tries to pull Not So Different on Chalky to gain his loyalty). Both come from "poor but honest" backgrounds and have little to nil formal education, and don't like it when people look down on them for these features. They're also both feared and respected by the men they work with, but dominated (literally, in Gyp's case...) by strong women at home. However, whereas Chalky ultimately respects education and is only involved in crime to provide a good life/social mobility for his family, Gyp loves being a gangster, and plays out his resentment by sadistically toying with other people.
As of the end of Season 3, Eli has Nucky's respect and is closer to being his full-on partner than ever before; in contrast to Jimmy and the Commodore, both of whom are dead, Eli has, in a roundabout way, gotten exactly what he joined the conspiracy in season 2 looking for.
Paul Sagorsky is a textbook example of the Hollywood take on the Vietnam War Vet (nowadays being superseded by the First Gulf War Vet). Of course, the show takes place decades before either, in the 1920s, so he is a veteran of the Philippine-American War. This is not a gratuitous choice, however: Although forgotten today, the Philippines war was very polemic at its time and the criticism about the aims and conduct of the US Army was in many ways identical to the one in 'Nam years later, to the point that Teddy Roosevelt of the "walk slowly and wave a big stick" himself had to put an official end to the hostilities in 1902 to deflect some of this criticism (actual fighting continued until 1913).
Richard comparing himself to the Tin Woodsman works more than one level. Like the Tin Woodsman, he has a prosthetic. But furthermore, the Woodsman claimed that now that he has no heart, he doesn't feel emotions. Richard similarly claims to be emotionless after his injury; he says he felt nothing towards his sister anymore, and that people cannot be really connected to each other. But the Woodsman wasn't really without emotion; he was actually the kindest character in the book. Richard too finds his humanity again during the series.
Frustrating as it is to see our favorite characters killed off, it makes sense; only the historical characters can survive long enough to make a name for themselves. This series is essentially about the gangsters who didn't make it, who vanished from history's memory.
Two things from Dean O'Banion's murder are noticeable in hindsight and they're somewhat disturbing:
When Frankie Yale walks in, he says, "What do you got that says 'you're sorry, and won't ever do it again'?" Yale is talking about murdering O'Banion here. To O'Banion himself. And it means two things: for one, Yale is a Professional Killer who has no personal beef with O'Banion, and won't ever do it again because he can't murder the same person twice. Two, in real life, O'Banion was killed after overstepping his bounds, pissing off the wrong people and alienating too many of his friends/allies. So Yale's basically mocking O'Banion's own situation and saying that no apologies are going to get O'Banion out of it, all without O'Banion realizing it.
Chrysanthemums would mean something completely different to O'Banion and Yale. To Illinois native O'Banion, chrysanthemums are cheerful flowers, so he offers them for Yale's wife. To someone born in southern Europenote Longobucco, Italy to be specific, in Yale's case, like Yale, chrysanthemums mean death. Fittingly, Yale deposits a chrysanthemum over O'Banion's body after killing him.
Narcisse agrees to take 10% of Chalky's club as compensation for Dickie Pastor's death, in exchange for "the other problem" going away. As they exit, he tells Alma Pastor that he is going to take her to "a place far away".
Just why is Gillian so creepily sure that Tommy will forget about Angela and Gillian herself and is so desperate to keep the children to her side, even if she's smothering them? It's because Gillian did forget her own parents. She was orphaned very young and was raised in a Dickensian orphan house in Trenton. She has no memory of them.
For Mafia history buffs, the fictionalized versions of real gangsters can invoke this, when one recalls specific crimes some of them were known for.
The comical scene where Bugsy Siegel sings "My Girl's Pussy" in "Friendless Child" takes a very dark tone when you realize that by this point (1930), Bugsy Siegel has committed at least one rape (as of 1926).