When James Sikking auditioned for the role of the assassin, Sir John Boorman rejected him and told him that his face was too nice for a killer. For the next week, though, Boorman would look out his office window at MGM and see Sikking standing outside, partially concealed by a bush or a column, just watching him menacingly. The director eventually walked out and offered him the part.
Lee Marvin didn't think John Vernon was good for the role, as the actor "wasn't strong enough to contend with him"; and during filming, Marvin punched Vernon in the stomach during a fight scene, causing Vernon to cry out and protest that he was an actor, not a fighter. But Vernon followed it with a visibly-increased energy and anger.
This was the first film shot at Alcatraz, the infamous prison which had been shut since 1963, only three years before the production. Two weeks in the abandoned prison facility required the services of 125 crew members. MGM agreed to pay $2,000 per day ($18,900 in 2024) to rent the prison and to pay all expenses to re-establish power, water, and heat necessary for the production.
Sir John Boorman was called in for a meeting with MGM President Robert O'Brien in which the executive immediately began expressing concern, but partway into the meeting the phone rang. It was David Lean making requests in advance of his next film, Ryan's Daughter (1970), and O'Brien was so excited to speak with him that when the call ended, he simply ushered Boorman out, saying only "make a good one". Boorman told Lean years later how he had saved him.
On the film's commentary, Steven Soderbergh talks about the sound of Walker's footsteps as he walks quickly through the airport, and Sir John Boorman adds that when Lee Marvin died, his widow asked the filmmaker if he wanted anything to remember him by, "and I took the shoes from that scene".