The director chose many of the locations due to their history of criminal activity. The major hotel they had shot in, was reportedly a hide out for famed bank robbers, Bonnie and Clyde.
The Rains character is loosely based on a Korean War veteran who went to work for the Purple Gang as an "Enforcer".
Rains' wardrobe was intentionally modeled after that of the Invisible Man in the 1933 classic.
Many of the thugs in the opening gun battle were ex-cons. The producers had to verify that shooting the guns in the film didn't interfere with any of their parole requirements.
During the scene where Ash shoots Rains, the authorities was notified that firearms were being used. Apparently, the information wasn't passed on to the next shift and cops showed up to the location with their guns drawn.