Sir Michael Caine was originally offered the role of Devlin, but did not want to play an I.R.A. member, and requested to play Steiner instead.
To research the role of Heinrich Himmler, Donald Pleasence spent hours at the Imperial War Museum in London watching footage of him and memorizing Himmler's gestures, walk and facial expressions.
In his autobiography, Sir Michael Caine confessed to being somewhat disappointed with the end product: "The picture was being directed by the Hollywood old-timer John Sturges, and we were all very pleased that this illustrious veteran had agreed to direct our film. That is, until one day when I was talking to him between set-ups and he informed me that, now that he was older, he only ever worked to get the money to go fishing, which was his passion. Deep-sea fishing off Baja, California, he added, which was very expensive. The moment the picture finished, he took the money and went. Producer Jack S. Wiener later told me that he never came back for the editing nor for any of the other post-production sessions that are where a director does some of his most important work. The picture wasn't bad, but I still get angry when I think of what it could have been with the right director. We had committed the old European sin of being impressed by someone, just because he came from Hollywood."
The watermill where the girl gets rescued is the one on the cover of Black Sabbath's first album. It's in the village of Mapledurham, Berkshire, England. The church where the hostages are kept is just across the road from it.
A funny thing about having a small stunt team: one of the German soldiers shoots and kills the driver of an American Jeep, so it crashes into the pond. Both the German and the driver of the Jeep were played by the same stuntman (Jim Dowdall). He kills himself, so to speak.