Dyeing for Your Art: Natural blonde Darren Nesbitt's hair is such an unusual shade because it had to be dyed twice. He was supposed to get it dyed back to blonde from another color (he often appeared with brown or black hair in films). But an accident with the process turned his hair green, forcing them to dye it again, turning it that unnatural-looking shade of platinum blonde.
Enforced Method Acting: Nazi concentration camp survivor Ingrid Pitt found filming very difficult because many of the cast were wearing World War II German army uniforms.
So My Kids Can Watch: Specifically written by Alistair MacLean to give his mate Richard Burton a good old-fashioned adventure romp to show the kids (said kids wanted to see their dad in something that wasn't highbrow and where he didn't get killed).
Troubled Production: The production was delayed while filming due to the weather in Austria. Shooting took place in winter and early spring of 1968 and the crew had to contend with blizzards, sub-zero temperatures and potential avalanches. Further delays were incurred when Richard Burton, well known for his drinking habits, disappeared for several days with his friends Peter O'Toole and Richard Harris.
Wag the Director: Clint Eastwood refused to have his hair cut for his role. He initially opined that the script written by Alistair MacLean was "terrible" and was "all exposition and complications", and-according to Derren Nesbitt-requested that he be given less dialogue. Most of Schaffer's lines were given to Richard Burton, whilst Eastwood handled most of the action scenes.
Lee Marvin was offered the role of Schaffer, but he declined, telling the producers they were about four years too late. Marvin had already starred in a WW2 action-adventure, The Dirty Dozen, which he hated. Although it made him a huge star, he did not want to return to that type of movie.
Michael Caine later claimed to have turned down the role of Major Smith.