The lead role was originally to have been played by Al Pacino, while Robert De Niro was to have played the driver Paulie in The Godfather (1972). Francis Ford Coppola wanted Al Pacino to play Michael Corleone, so to get Pacino released from his contract for this film, he had to give up De Niro to replace him.
To prepare for his role, Robert De Niro went to Florida to watch baseball teams in spring training, then travelled to Georgia and tape recorded conversations to study their accents.
The television performance of the "Singing Mammoths" was also shot at Shea Stadium on the set of "Kiner's Korner," the post-game show of New York Mets telecasts, hosted by Met broadcaster, and Hall of Fame outfielder, Ralph Kiner.
The character Henry Wiggen originally appeared as the hero in the novel, "The Southpaw," which was written by Mark Harris. Mr Harris wrote the novel upon which this film is based as well as the screenplay.Henry Wiggen, however, as played by Michael Moriarty, is not a southpaw, but a right-handed pitcher.