Tony Curtis was generally 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.