* TheDanza: Mary Ure as Mary Elison.
* DyeingForYourArt: Natural blonde Derren 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.
* EnforcedMethodActing: [[UsefulNotes/TheHolocaust Holocaust]] survivor Creator/IngridPitt found filming very difficult because many of the cast were wearing UsefulNotes/WorldWarII German army uniforms.
* HeyItsThatPlace: The Austrian castle, Schloss Hohenwerfen, is the same castle that can be seen in the background in a scene from ''Film/TheSoundOfMusic'' when Maria and the kids are singing "Do Re Me".
* MoneyDearBoy:
** Creator/ClintEastwood was reluctant to receive second billing to Burton, but agreed after being paid $800,000.
** A persistent tale is that Burton needed money to pay either the taxman or his bar tab, so [=MacLean=] wrote the screenplay over a weekend as a money-spinner. This isn't true, but is a good story.
* OnSetInjury: Derren Nesbitt was injured whilst filming his death scene. The blood squib attached to him exploded with such force that he was temporarily blinded, though he made a quick recovery.
* ProductionNickname: Creator/ClintEastwood nicknamed the film "Where Doubles Dared" due to the amount of screen time in which stand-ins doubled for the cast during action sequences.
* SelfAdaptation: Creator/AlistairMacLean wrote the script, thus adapting his own novel.
* SoMyKidsCanWatch: Specifically written by Creator/AlistairMacLean to give his mate Creator/RichardBurton a good old-fashioned adventure romp to show the kids, who wanted to see their dad in something that wasn't highbrow and where he didn't get killed.
* StuntDouble: Famed stuntman Alf Joint (Capungo in ''Film/{{Goldfinger}}'') was stand-in for Creator/RichardBurton, and performed the famous cable car jump sequence, during which he lost three teeth. Joint stated that at one point during production, Burton was so drunk that he knocked himself out while filming and Joint had to quickly fill in for him.
* TroubledProduction: 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 Creator/RichardBurton, well known for his drinking habits, disappeared for several days with his friends Creator/PeterOToole, Creator/TrevorHoward and Creator/RichardHarris. According to Derren Nesbitt, Burton was drinking four bottles of voldka a day. At one point, Burton was threatened by a jealous husband with a gun in a bar, leaving Creator/ClintEastwood and Creator/ElizabethTaylor to deal with him. Nesbitt, when filming von Hapen's death scene, had to be rushed to the hospital after a squib malfunctioned and went off into his eyes, nearly blinding him.
* WagTheDirector: Creator/ClintEastwood refused to have his hair cut for his role. He initially opined that the script written by Creator/AlistairMacLean 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 Creator/RichardBurton, whilst Eastwood handled most of the action scenes.
* WhatCouldHaveBeen:
** Creator/LeeMarvin 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, ''Film/TheDirtyDozen'', which he hated. Although it made him a huge star, he did not want to return to that type of movie.
** Creator/MichaelCaine claimed to have turned down the role of Major Smith. He would play the lead in a UsefulNotes/WorldWarII commando film seven years later, ''Film/TheEagleHasLanded'' -- albeit as a German disguised as a Briton, the opposite of Smith.
* WorkingTitle: ''Castle of Eagles''.
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