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Tony Sirico only agreed to sign on for the show if it was guaranteed that his character Paulie "Walnuts" Gualtieri would not be a "rat" (an informant). As Sirico explained in James Toback's documentary The Big Bang (1989), he had served time in prison for robbery. Altogether, Sirico's rap sheet included at least twenty-eight arrests. Reportedly, he appeared briefly in an uncredited role in The Godfather Part II (1974). Some aspects of Sirico's real life, a brief stint in the military, et cetera, were added to Paulie's life as well.
James Gandolfini said that he was often contacted by real-life "wise guys," complimenting him on the authenticity of the series, as well as giving him advice.
It is said that during some scenes, James Gandolfini inserted a small stone in his shoe to anger him, making him play the role of Tony Soprano more authentically. He would also stay awake all night for some of the breakfast scenes, to achieve a tired look.
After the pilot aired, a real-life "wise guy" told James Gandolfini never to wear shorts again. The encounter seems to have been incorporated into season four, episode one, "For All Debts Public and Private," when New York City mob boss Carmine tells Tony that he'd heard about his recent backyard party, and that "a don doesn't wear shorts." However real life ex-mob capo and street boss Michael Franzese disputes this claim whereas his father Sonny wore shorts on a regular basis.
To settle salary disputes after season four, James Gandolfini gave each main cast member $33,333 from his own pocket.