The sound track used on this movie is the same one written by Malcolm Arnold for the movie of a true story made in 1958, called The Inn of the Sixth Happiness (1958) starring Ingrid Bergman as Gladys Aylward the English missionary to China who rescued at least 50 children from the village that was bombed by the Japanese. Based on her own book.
According to Kirk Douglas in his autobiography, the German commandant of the ship carrying the heavy water train wagons in the final climax sequence is the actual former commanding officer who was in charge of this very same ship during WW2.
In November 2009, it was reported that the World War II raid on the Telemark hydro station is to be re-enacted by a Royal Marines unit who intend to ski across seven hundred miles of frozen Norwegian landscape to raise money for charity.
The book 'The Special Operations Executive 1940-1946' by M.R.D. Foot states that Operation Gunnerside, one of the three and most successful of the three missions that this film is based on, was later labeled by the British Special Operations Executive (SOE) as being the most successful act of sabotage in all of the Second World War. This raid was given the highest military priority and it dealt a serious drawback to the Nazis' development of an atomic bomb in World War II.
Although this movie takes liberties with some historical facts, some technical details are surprisingly correct given that it was made 20 years after the war: The car used by the Norwegians is fitted with a "generator" converting wood to natural gas. As petrol was in short supply, civilian cars were not allowed to run on real petrol.
The real life World War II missions that this film is based on were conducted by Norwegian members of the British Special Operations Executive (SOE) and resulted in the deaths of thirty British commando soldiers. Some were captured, interrogated, tortured and shot by the Nazi Gestapo whilst some of them died when two gliders crashed during landing in Norway.
The real mission code-names for the Telemark operations in World War II were Operation Freshman, Operation Grouse and Operation Gunnerside. The raid was originally scheduled for October 1942 but was not conducted until February 1943, four months later. Operation Grouse established an advance team of four Norwegians above the hydro plant at the Hardanger Plateau. Operation Freshman was unsuccessful and resulted in the deaths of British paratroopers when gliders crashed and any survivors were captured, interrogated, tortured and shot by the Nazi Gestapo. Its aim was to rendezvous with the Norwegians from Operation Grouse and proceed to and then raid the Vemork hydro station. Operation Gunnerside was the successful second attempt of Operation Freshman.
The Telemark of this movie's title refers a county district in the region of Østlandet (East Country / Eastern Norway) in Norway. The county was formerly known as Bratsberg amt up until 1919. Telemark borders the Norwegian counties of Vestfold, Buskerud, Hordaland, Rogaland and Aust-Agder. Telemark is the county where the real life World War II mission to raid the Vemork hydroelectric heavy water plant in the town of Rjukan (located in Tinn, a municipality in Telemark) took place and which forms the basis for this movie.
During the filming of Heroes Of Telemark, Benton Films were using the large stone church Rjukan Church. One night the whole interior of the church burnt down in a raging fire. Only the solid stone walls were standing. It was a large scandal, of course, where the church lost some great reliquaries impossible to replace. The reason for the fire is supposed to be light equipment. It took three years to rebuild the church after the fire.
A documentary about the mission called The Real Heroes of Telemark (2003) was later made two years shy of forty years after this movie was made. This documentary was apparently made to clear up some of the fictionalization of this movie which portrayed some inaccuracies, variations and differences compared to the events and circumstances of the true World War II Telemark raid.
Seventeen years prior to this film, a Norwegian-French movie Kampen om tungtvannet (1948) was made which covered the same events of the World War II Telemark mission as depicted in this film. This earlier movie actually featured some of the original Norwegian commandos who played themselves.