Robert Mitchum was originally cast in the lead, but shortly after filming began, he and director Otto Preminger had a major blow-up and Mitchum either quit (according to Mitchum) or was fired, drinking being a factor (according to Preminger). Peter O'Toole, a former drinker, was hired to replace him in about a couple of days. Reportedly. Mitchum likened this to being like "replacing Ray Charles with Helen Keller."
Director Otto Preminger's son Erik Lee Preminger wrote the screenplay. This was the only script that the younger Preminger wrote. Neither Preminger knew of their relationship until Erik reached adulthood, and the senior Preminger's impulse toward belated fatherly benevolence doomed the production.
A bomb threat made to Peter O'Toole where the Irish Republican Army called him "an alleged Irishman" - which turned out to be a hoax - shut down the production for a short period. It was communicated by way of a letter, which was revealed to be a prank by movie critic Kenneth Tynan. The joke angered two crew members who allegedly went and beat up Tynan.
The screenplay was constantly being re-written throughout the production.