Jack Nicholson was to play Bruce Dern's character, Johnny May, but instead shows up in a bit part as a henchman, Gino, loading garlic-soaked bullets into a Tommy gun (Nicholson was still paid for all seven weeks of the shoot.)
Orson Welles was originally picked by director Roger Corman to play Al Capone, while Jason Robards was to play George Moran ("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.
Although most of the facts in the film are close to the truth, the only real deviation concerns the fate of Joe Aiello. While the movie does accurately portray Aiello aligning himself with George Moran and conspiring to kill Mafia chieftain Pasqualino "Patsy" Lolordo, he was not killed personally by Al Capone on a train (though Capone was known to kill when seized by a fit of personal rage). As shown in the film, Aiello, knowing he was marked for death by Capone, did arrange to have a cousin purchase a train ticket for him at the last minute so that he could skip town. However, he was killed by machine gunners from an apartment window opposite his apartment building as he was leaving to catch the train on October 23rd, 1930: A year and nine months after the massacre, not before.
At the beginning of the movie when Peter Gusenberg (George Segal) asks the barkeep where he's getting his beer from, the barkeep answers, "A fellow named Slausen", to which Gusenberg replies, "Slausen? The only Slausen I know works for Caponi, Al Caponi." Believe it or not, Gusenberg's pronunciation of Al Capone's name is in fact a source of debate amongst historians. Though he's known as "Capone" with an "E", early arrest sheets and Chicago Tribune articles listed Capone's name as "Caponi" with an "I". However, the Chicago Tribune was known at the time for their blatant spelling errors (like "clew" for "clue") and may be responsible for this misconception of Capone's name.
Average Shot Length = ~9.2 seconds. Median Shot Length = ~8.4 seconds.
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.
More squib charges were used in this film than in the three-hour war epic The Longest Day.
The film came in at $200,000 under budget because Roger Corman reused sets from other movies, including a mansion that served as Capone's home (even though in reality Al Capone lived in a modest brick home in a working-class neighborhood).
The movie's mostly historical accuracy also includes some of the real words spoken by those involved in the massacre. Frank Guesenberg's dying answer to the police officer's inquiry really was, "Nobody shot me. Leave me alone". George Moran really did exclaim to a reporter, "Only Al Capone kills like that." Capone responded to the press: "Only Moran kills like that. I mean, they don't call that guy 'Bugs' for nothing!"
The real garage where the massacre took place (2122 N. Clark St, Chicago) was torn down three months after the movie was released.
The set used as a brothel also served the same function in Fox's The Sand Pebbles
This was Roger Corman's first directorial project for one of the major studios (Twentieth Century-Fox). After approximately 15 years experience as a producer/director of low budget productions, it was no surprise that Corman wrapped this production ahead of schedule and well under budget.
When "Bugs" Moran is leaving his hotel, just before the massacre, the clerk stops him and says "You have a call from a Mr. Bernstein in Detroit". "Bernstein" was the last name of the four brothers - Abe, Joe, Ray and Izzy - who ran Detroit's murderous "Purple Gang". The real George Moran and Al Capone used the Bernsteins to hijack other gangsters' liquor shipments from Canada. This fact is evident later when Moran complains to his bodyguards about Bernstein jacking up the price.