Tony Curtis generally was considered too old to play Albert DeSalvo despite being only six years older than DeSalvo. At the times of the murders, DeSalvo was only in his early thirties.
Shortly before filming was set to begin, the real Albert DeSalvo escaped from the mental institution where he was imprisoned. He was later captured after being on the run for thirty-three hours.
According to TCM's Ben Mankiewicz, Tony Curtis, with his career in decline at the time, lobbied 20th Century-Fox boss Richard D. Zanuck for the DeSalvo role but was rebuffed as Zanuck could not see past Curtis' history of playing comedies and romantic leads. So Curtis hired a makeup artist and a photographer and had several "mug shots" done and sent to Zanuck - who didn't recognize the actor at all. That effort got Curtis the job.
According to an article in the Februrary 1969 edition of American Cinematographer, this was the first Hollywood feature film to use the "multiple image" technique of presenting several scenes in panels on the screen simultaneously. The author estimated 35% of the film's total footage was used this way. The article has been reprinted online at the publication's web site on 20 May 2020. It includes extensive information from director Richard Fleischer, director of photography Richard H. Kline, editor Marion Rothman, and special effects photographer L.B. Abbott.