* AbilityOverAppearance: Most of the actors portraying historical or semi-historical characterd share little resemblance (or none whatsoever) with their historical counterpart
* AccidentallyCorrectWriting: At the time the show was made, Nan Britton's claim to have had a baby with Warren G. Harding was considered a lie. But shortly after it was portrayed, new DNA evidence actually backed it up.
* ActorInspiredElement: Jack Huston came up with Harrow's guttural voice himself, using Creator/MarlonBrando's cotton-in-the-mouth trick from ''Film/TheGodfather''.
* DawsonCasting:
** 37 year old Stephen Graham as 21 year old UsefulNotes/AlCapone.
** Also, 30-year-old Michael Pitt as 23-year-old Jimmy Darmody.
** [[http://www.imdb.com/name/nm0992195/ Anatol Yusef]] as Meyer Lansky. No ''way'' is he 18.
** 62 year-old Greg Antonacci as ''38'' year-old Johnny Torrio.
* FakeNationality:
** The very German Eddie Kessler is played by Anthony Laciura, who is of Italian descent. The casting crew was in fact surprised when they learned that Laciura, who had made his entire introduction in a German accent, was actually an Italian-American from the Bronx.
** The Italian Gyp Rosetti is played by Italian-American Bobby Cannavale
** FakeAmerican: Englishmen Stephen Graham, Anatol Yusef, Jack Huston, Marc Pickering, and Ian Hart as UsefulNotes/AlCapone, Meyer Lansky, Richard Harrow, young Nucky Thompson, and young Ethan Thompson, respectively.
** FakeIrish: Scottish Creator/KellyMacdonald and Englishman Creator/CharlieCox as Margaret Rohan Schroeder/Thompson and Owen Sleater. Also, Scot Creator/TonyCurran as Margaret's brother Eamon and American Ted Rooney as the Sinn Féin politician John [=McGarrigle=]. Irish-American Nucky Thompson is played by Italian-American Creator/SteveBuscemi who is of half-Irish descent.
** Also, the French Isabelle Jeunet is played by Swiss actress Anna Katarina.
** Washington-born Jeffrey Wright plays Trinidarian gangster Valentin Narcisse.
* TheOtherDarrin: The role of Will, Eli's oldest son, was recast for Season 4.
* PlayingAgainstType:
** While Steve Buscemi is no stranger to playing criminal characters, he's always been weaselly supporting characters, not the suave man in charge.
** Michael Stuhlbarg, best known up to this point for playing [[ButtMonkey Larry Gopnik]] in ''Film/ASeriousMan'', now plays Arnold Rothstein, a ruthless mobster. After this series, Stuhlbarg received more roles as criminals and dangerous schemers.
** Perennial bedwetter Michael Pitt as certified hardass Jimmy Darmody.
** Creator/JimTrueFrost's appearance in season 5 as the most famous no-nonsense cop in history Eliot Ness is hilarious to anyone who remembers him as the perpetual screw-up Roland Pryzbylewski on ''Series/TheWire''.
* RoleEndingMisdemeanor: Anonymous reports from crew members claim both Michael Pitt and Paz de la Huerta were written off of the show for prima donna behavior on the set, with Lucy being PutOnABus and Jimmy being KilledOffForReal. Showrunner Terence Winter has strongly denied that Pitt was fired for on-set behavior, however.
* SerendipityWritesThePlot:
** Terence Winter and Dabney Coleman confirmed that the Commodore's stroke was written in when Coleman was diagnosed with throat cancer, whose treatment made him hard to speak, and as a result [[DiscOneFinalBoss sidelined]] what was first intended to be a [[WhatCouldHaveBeen large part]] as the season's BigBad. This had far reaching consequences in the show's storyline, since it pushed Jimmy to the front of the conspiracy, and made it impossible for Nucky to realistically forgive him. As a result, Jimmy's (and by extension, Angela's) storyline had to be cut earlier than expected, with Gillian and Richard being brought to the front in Season 3 to fill the gap.
** Creator/CharlieCox said that they wrote a fight scene for Owen's death in "A Man, A Plan..." but they [[NoodleIncident didn't shoot it]] in the end. According to Winter, this was because they had to suspend filming when a chunk of plaster from the ceiling of the old baths were they were going to shoot that scene fell off. In the meantime the rest of the episode was put together and, after watching it, they realized that not seeing the fight actually made TheReveal at the end of the episode, when Owen's body is shipped back to Nucky in a box after the hit's failure, more powerful.
* WhatCouldHaveBeen:
** Creator/JamesGandolfini was the first choice to play Nucky Thompson. Winter remarks that he had an uncanny resemblance to the actual Nucky, but having just finished his stint as [[Series/TheSopranos Tony Soprano]], he opted out to pursue other roles and to avoid being typecast. Creator/AlecBaldwin was also considered.
** The first casting calls for Season 1 called for the introduction in the season finale of "Declan", Margaret's long estranged brother who was now linked to the [[UsefulNotes/TheTroubles IRA]]. Instead, Season 2 divided this character and introduced separately Margaret's brother Eamonn, a Brooklyn subway-digger with no interest in politics, and Owen, an IRA enforcer completely unrelated to Margaret.
** Creator/AleksaPalladino revealed in an interview that before Jimmy had to be killed off, one considered plot line for the third season involved Angela and Jimmy moving to New York City, and Angela meeting a female activist that would introduce her to Anarchism.
** The first casting calls for Al Capone's brothers Frank and Ralph came out in Season 2, but Domenick Lombardozzi (seen in ''Series/TheWire'' and ''Series/{{Entourage}}'') was confirmed as Ralph only in Season 4. In Season 2 Al mentions that he is going to Brooklyn to take his brothers and bring them to Chicago.
** Shortly after the Season 2 finale there was a very talked about call for (and even leaked auditions of) a new "young", "sexy" recurring gangster character named Bud Matheson that was [[SuspiciouslySimilarSubstitute perhaps too reminiscent of Jimmy]]. A similar call for a "young", "sexy" college freshman called Michael [=McHugh came=] after Season 3. So far, neither seems to have made the cut in the end, though elements of both likely found their way into [[OneSceneWonder one-shot character]] Rowland Smith and [[RememberTheNewGuy Eli's eldest son, Willie]].
** If Dabney Coleman had not had throat cancer, the Commodore would not have been incapacitated early in Season 2 by a stroke and remained the season's main antagonist. It's likely that the season would end with Jimmy killing the Commodore at the request of Nucky (as WordOfGod is that he would die in the season in every case), instead of Jimmy doing it on his own volition, and Nucky allowing Jimmy to live if he left Atlantic City (since the Commodore, not Jimmy, would be the one to order Nucky's assassination).
** Two seasons and a good chunk of another were excised because of the show's early cancellation. One can only imagine how Eli and Van Alden managed to survive the Chicago gangster wars, how and why Rothstein was killed in the show, or what part Nucky might have played in Machado's Cuba and the Castellammarese War.

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