In the scenes where Richard Burton and Clint Eastwood climb the steep fortress walls, Burton moves with ease, while Eastwood is clearly working hard physically. This was due to the fact that Burton, who was a hard-drinker, a chain smoker and out-of-shape by that point, chose to ride a crane (made invisible by special effects) up the wall, whereas the health-conscious Eastwood was actually climbing the wall. Burton had already been diagnosed with bursitis - possibly aggravated by faulty treatment - arthritis and dermatitis.
The part that ultimately went to Clint Eastwood was also offered to Lee Marvin, but he declined, telling the producers they were about four years too late. Marvin had already starred in a World War II action-adventure, The Dirty Dozen (1967), which he hated. Although it made him a huge star, he did not want to return to that type of movie.
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.
The driving force behind the film was Richard Burton's stepson, who wanted to see his stepfather in a good old-fashioned adventure movie. Burton approached producer Elliott Kastner for ideas, who asked Alistair MacLean. At that time, most of MacLean's novels had either been made into films, or were in the process of being filmed. Kastner persuaded MacLean to write a new story. Six weeks later, MacLean delivered the script.
The JU-52 (HB-HOT) used in the movie as the rescue aircraft at the end, crashed into Piz Segnas, a mountain in the Swiss Alps, on August 4, 2018, killing all 20 on board. It was on a flight from Locarno to Dubendorf in Switzerland.
Clint Eastwood 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.
The "Schloss Adler" is actually the "Schloss Hohenwerfen" in Austria. At the time of filming, the castle was being used as a police training camp. There are no cable cars near Schloss Hohenwerfen. Hence the Cable Car shooting is done somewhere else.
Alistair MacLean wrote the script first and then the novel immediately afterward. Although the underlying plot remains the same, the book and script are not entirely faithful to one another. For instance, the book is substantially less violent and the characters are somewhat more comedic. Also noteworthy is the book included a brief love story involving Schaeffer and Heidi.
In a recent Channel 4 (UK) survey of the top 100 war movies Steven Spielberg voted this as his favorite, mainly due to its sheer "boys own" factor of unreality. He even went so far as to repeat the "Broadsword calling Danny Boy" line.
As part of his deal with MGM, Clint Eastwood took delivery of a Norton P11 motorcycle, which he 'tested' at Brands Hatch racetrack, accompanied by Ingrid Pitt, something that he had been forbidden from doing by Elliott Kastner for insurance purposes in case of injury or worse.
The Junkers Ju 52 used to fly Smith and Schaffer's team into Austria and then make their escape at the end of the film was a Swiss Air Force Ju-52/3m, registration A-702. It still wore the "Where Eagles Dare" camouflage pattern in 1981, as various photographs show. In 1982, it was sold to a private operator, and still flies as HB-HOT.
The name Mary is given when introduced as Heidi's cousin, is Maria Schenk. Maria Schenk was the middle name of Colonel Claus Philipp Maria Schenk Graf von Stauffenberg who was the chief conspirator in the July 20th plot to assassinate Adolf Hitler. It was Stauffenberg who placed the briefcase with the bomb under the table at the Wolf's Lair.
Derren Nesbitt was keen to be as factual as possible with his character Von Hapen. Whilst on location, he requested to meet a former member of the Gestapo to better understand how to play the character and to get the military regalia correct.
On location in Austria, Richard Burton was drinking in the hotel bar with Elizabeth Taylor and Clint Eastwood, when a jealous husband came and pressed a gun against his stomach. Confident that Eastwood and Taylor could handle the man, he excused himself to go to the lavatory. He came back to find the man gone and Taylor purring in triumph.
Wilhelm Scream: When the car carrying Richard Burton, Clint Eastwood and the German captors crashes into the snow following an attempt to kill the captors. Wilhelm Scream heard as the German flies through the car window.
Even though Alistair MacLean wrote both the book and the film, several characters have different names in the book and the film respectively. Curiously some sources (e.g. Screen World) refer to these names in the cast list.
As of December, 2018, the only (billed) cast members currently living are: Clint Eastwood (Schaffer) and Derren Nesbitt (Von Hapen). Furthermore, both men are still working actors with features released in 2018: "The Mule" and "Tucked"
Col Turner is wearing a different uniform in the opening briefing scene compared with the final airplane scene. His rank on the shoulder straps in the briefing scene are metal badges, with the shoulder strap attached with a metal button, yet in the final scene the button is missing and the rank on the shoulder strap is embroidered.
A previous poster stated incorrectly that Lieutenant Schaeffer was wearing a "Senior Parachutist Badge" on his uniform during the flashback briefing sequence. Neither "Senior" jump wings, with a star on top the parachute canopy, nor "Master" jump wings, with a star and wreath, existed during World War II. Schaeffer is in fact wearing the only jump wings that existed during WWII, later designated as "Basic" wings after the post-war institution of the Senior and Master parachutist badges. As mentioned by other posters, he is wearing them over the wrong (right) chest pocket flap of his uniform. They properly belong over (or on) the left pocket flap.
The trivia items below may give away important plot points.
Derren Nesbitt was nearly blinded when the squibs in his chest blew upwards instead of outwards when filming his death scene - his character was filmed being shot in the head and the chest but in the finished film he is only shot in the head.
It is a common myth that no German character with a speaking role survives until the end of the film. However this is untrue, at very least the officer who tries to phone the airfield to warn them lives.