Residents of the area around Stockton, California, were used as extras in the film; often, director Robert Rossen would give them speaking parts and film the "rehearsals" to get a more spontaneous effect.
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.
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 derrière . . . " Wayne later remarked that "To make Huey Long a wonderful, rough pirate was great . . . but, according to this picture, everybody was shit--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 Wayne's Seven Sinners (1940), eventually received the part of Stark. In a bit of irony, Crawford was Oscar-nominated for the part of Stark and found himself competing against Wayne, who was nominated the same year for Sands of Iwo Jima (1949). Crawford won the Best Actor Oscar, giving Rossen the last laugh.
Director Robert Rossen filmed in an unusual manner. Nobody in the cast had a script. Rossen let them read it once and took it away from them. According to Broderick Crawford, "We really had to stay on our toes."
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' (qv() to Willie Stark.
In the original manuscript, Willie's last name is Talos, not Stark. When the novel was first published, the name was Stark. In 2001, the novel was reprinted with Stark's name changed back to Talos, among other changes.