Trivia

For the massacre scene in the garage, the actors playing the slain gangsters were shown photos and directed as how to fall so their positions were identical to the real photos of the massacre. Two actors bumped together on the way down. After studying photographs they realized they had fallen and collided in the exact way the slain gangsters had fallen and had landed in the correct positions.

Orson Welles was originally picked by director Roger Corman to play Al Capone, while Jason Robards, Jr. was to play George "Bugs" Moran. Welles was willing but Fox vetoed the deal, feeling Welles was "undirectable". Robards took over the role of Capone and Ralph Meeker was brought in to play Moran.

Jack Nicholson was to play Johnny May (Bruce Dern), but instead shows up in a bit part as a henchman, Gino, who explains loading garlic-soaked bullets into a Tommy gun (he was still paid for all seven weeks of the shoot).

The real garage where the massacre took place (2122 N. Clark St., Chicago, IL) was torn down three months after the movie was released.

Although the film came in for $2.1 million--having originally been budgeted for $2.5 million--and was thus by far the most expensive film of Roger Corman's career to that time, he insisted that he could have shaved an extra $1 million off the budget (without changing the script or hiring different and less expensive actors) if he had been allowed to make the film all on Chicago locations with a mostly non-union crew instead of in the Hollywood studios. He also finished the film four days ahead of schedule. He didn't forget his unofficial stock company and cast some of them in this film, including Dick Miller, Jonathan Haze, Betsy Jones-Moreland, Barboura Morris and Jack Nicholson.