The first cut of the film was considered disastrous by all involved. Editor Ralph Rosenblum worked for more than a year to save it, with director William Friedkin long gone. The extensive use of period film clips was Rosenblum's idea. The technique of returning from these clips to the movie by starting with a black-and-white version of a shot and changing to color was invented accidentally when the editor's assistant couldn't find the color copy of a piece of film fast enough.
Tony Curtis was offered the role of Raymond Paine but declined due to disagreements over the script and was replaced at a month's notice by Jason Robards. The role of Billy Minsky was offered to a young Alan Alda but he was unavailable due to his commitment to "The Apple Tree" on Broadway.
Because of the excessive overtime generated by shooting around the death of Bert Lahr, Norman Lear gave gifts of initialed Tiffany silver money clips to many of the crew members, at the wrap party.
Burlesque legend Joey Faye was stand-in for the incomplete scenes of Bert Lahr, who passed away during filming.
The opening scene shows Rachel Schpitendavel Britt Ekland riding the 'ell' (New York's elevated trains) in Manhattan to the lower east side area. As the last ell in Manhattan had been dismantled in the late 50's (the 3rd Avenue el), the scene was shot on what was (and is still) a integral part of NYC's subway system; the above-ground lines running through parts of Brooklyn. The only difference between what was once known as the els and the above-ground lines, is that the els were lines in and of themselves, whereas the above-ground tracks are parts of lines which travel underground (i.e. the 'sub' - in 'subway') as well. When the els in Manhattan's east side were taken down, it left the entire east side (north-south) only transferable by either buses or the Lexington Ave. lines the N°s. 4, 5 & 6 trains, because trying to dig a subway line - in a city where space is at a premium - so difficult, that it took almost 75 years since work actually began to complete only a minor portion of the much-delayed Second Ave. line (it opened January,2017).