In the movie, the Gangster Squad is assembled to go after Mickey Cohen. However, in real life Chief W.H. Parker set up the squad to prevent other gangsters from taking over Cohen's rackets after Cohen went to prison on income tax evasion. Whenever the squad learned that out-of-town gangsters were coming to Los Angeles to try to set up new operations, the Gangster Squad would kidnap them, beat them up (some rumors say torture) and then send them back to wherever they came from with the warning that if they ever returned, they would be killed.
Ryan Gosling's exchange with Emma Stone early in the film about playing "post office" is lifted word-for-word from The Three Stooges short Three Little Pigskins (1934) in a scene with Curly Howard and Phyllis Crane. It also starred a young platinum blonde named Lucille Ball.
The last two surviving members of the original gangster squad came to visit the set, along with relatives of the deceased members.