The very German Eddie Kessler is played by Anthony Laciura, who is very American. 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.
Fake American: Englishmen Stephen Graham, Anatol Yusef and Jack Huston as Al Capone, Meyer Lansky and Richard Harrow, respectively.
Fake Irish: Scottish Kelly Macdonald and Englishman Charlie Cox as Margaret Rohan Schroeder and Owen Sleater. Also, Scot Tony Curran as Margaret's brother Eamon. Irish-American Nucky Thompson is played by Italian-American Steve Buscemi 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.
Tony Blundetto controls Jersey and clashes with Billy Leotardo (Tonino Sandrelli), who is once again the right hand of a hot-headed mobster. Phil Leotardo's consigliere? Johnny Torrio. Uncle Junior with sideburns counsels around, Nucky's father is played by Tony Soprano's father-in-law and Benny Fazio is now Leo D'Alessio. Mayor Bader coached Meadow's soccer team.
In a subversion, Steve Buscemi is no stranger to playing criminals, he's usually cast as weaselly supporting characters rather than the suave man in charge, when contrasting Nucky with some of Buscemi's other roles.
Michael Stuhlbarg, best known up to this point for playing Larry Gopnik in A Serious Man, now plays Arnold Rothstein, a ruthless mobster.
Perennial bedwetter Michael Pitt as certified hardass Jimmy Darmody.
Jim True-Frost 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 The Wire.
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 sidelined what was first intended to be a large part as the season's Big Bad. 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.
Charlie Cox said that they wrote a fight scene with Owen for "A Man, A Plan..." but they 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 The Reveal at the end of the episode more powerful.
James Gandolfini 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 Tony Soprano, he opted out to pursue other roles and to avoid being typecast.
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 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.
Aleksa Palladino revealed in an interview that one considered plot line for the 3rd season involved Angela and Jimmy moving to NYC, 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 The Wire and 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 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 one-shot character Rowland Smith and Eli's eldest son, Willie.