Producer-Director Robert Rossen offered the role of Willie Stark to John Wayne. Rossen sent a copy of the script to Wayne's agent, Charles K. Feldman, who forwarded it to Wayne. After reading the script, Wayne sent it back with an angry letter attached. In it, he told Feldman that before he sent the script to any of his other clients, he should ask them if they wanted to star in a film that "smears the machinery of government for no purpose of humor or enlightenment," that "degrades all relationships," and that is populated by "drunken mothers; conniving fathers; double-crossing sweethearts; bad, bad, rich people; and bad, bad poor people if they want to get ahead." He accused Rossen of wanting to make a movie that threw acid on "the American way of life." If Feldman had such clients, Wayne wrote that the agent should "rush this script... to them." Wayne, however, said to the agent that "You can take this script and shove it up Robert Rossen's derri?re..." Wayne later remarked that "To make Huey Long a wonderful, rough pirate was great," he said; "but, according to this picture, everybody was s - t except for this weakling intern doctor who was trying to find a place in the world." Broderick Crawford, who had played a supporting role in

Mercedes McCambridge's film debut.

To prepare for the role of Willie Stark, Broderick Crawford watched newsreels of Huey Long, the former Louisiana senator and governor on whose life the novel was loosely based.

Robert Penn Warren's novel, upon which the film was based, was published in 1946. It went on to win the Pulitzer Prize. Writer-director Robert Rossen purchased the film rights himself, and was then able to broker a deal with Columbia Pictures. He shifted the focus of the novel from the Jack Burden character (played by John Ireland) to Willie Stark.

Mercedes McCambridge was cast after she got angry with the producers. She and other actresses were kept waiting in an office in New York City during open auditions. McCambridge told the producers off and stormed out of the office. They called her back and eventually cast her because she fit the part of Sadie.