Anecdotes

James Whitmore debuted in this film in Chicago, Illinois, and on television on the same day - March 20, 1949 - in Dinner at Antoine's (1949) starring Steve Cochran, also in his television debut. Whitmore's next movie role, Bastogne (1949), earned him an Oscar nomination.

Based on the Internal Revenue Service Criminal Investigators who took down Al Capone after the Police and FBI were unable to make anything stick because of his ability to pay off or intimidate any witnesses that came forward - just like in this movie. The character of Frank Warren (Glenn Ford) represents real-life agent Frank J. Wilson, the lead agent who finally took down Capone, and the author of the autobiography "Undercover Man" that was serialized in Collier's magazine in 1947 and is the basis for this film.

Leo Penn, who played accountant Sydney Gordon, was the father of actors Sean Penn and Chris Penn.

While the film ostensibly takes place in Capone's Chicago (two decades after the fact), just exactly where its story takes place is never made explicit. Plenty of obvious (money saving) Los Angeles exteriors are used, the cars carry California license plates, and the young runaways frolic on the beach in Malibu. The tenement district and street vendors scenes were evidently shot on the studio's "New York City" back lot. The only actual clue given is that Warren's wife goes to stay at her folks' farm "a couple of hundred miles away" in "Tower City: Dairyland USA", potentially putting it to the north of Chicago in some of America's finest dairying.

The shyster's car is a 1947 Lincoln Continental Convertible Cabriolet (model 76H-56). Only 738 of this model were made that year. It sported a V-12 engine with a base price of $4,746 (about $69,200 in 2025). It was one of the most expensive domestic cars made that year. When Stan says the car must cost at least $5,000 to $6,000, he was correct - considering options, taxes, etc.

The police car that collects Warren at Malibu airport is a 1947 Ford Super De Luxe sedan. The prison vans are based on the 1941-6 Chevrolet Suburban. Judith Warren's parents own a 1931 Ford Model A roadster pickup.