If you watch the extras on the box-set, you'll see that 'Nucky Johnson' - the real life bootlegger and treasurer of Atlantic City - was a tall, burly, outgoing and domineering man.
His physical presence and charisma are what made him a successful politician and businessman.
Instead, Steve Buscemi's casting and portrayal of Nucky Thompson is a more softly spoken and less vivacious interpretation.
Nucky Thompson is shown as being intelligent, complex, self-doubting and paranoid. Although he is the guy who "owns" the town, we see a less outgoing and confident side to the character, that probably didn't exist with the real deal.
The other big difference between Boardwalk Empire and real life is the violence. While Nucky Johnson was certainly a criminal mastermind, he got his way by offering people jobs, bribing them, taking away jobs and making life financially difficult for those who opposed him.
He used his political and personal influence to control the town, whereas the TV series shows a world that is very much controlled by the gun.
