The original draft of the script was a serious drama, based on the true story of a group of allied POWs challenged to a football match by the Germans. The deal was that if the Germans won the match, the POWs would be set free in Switzerland. However if the POWs won, they would be shot. The POWs decided to go for 'victory', won the match and were consequently executed.
The movie was inspired by an actual series of games in Kiev, during the German occupation of the city. Several members of Dynamo Kiev, the top soccer team in Ukraine, found work in a bakery. There they formed a soccer team with other bakery employees. They began playing in a new league against teams supported by the Ukranian puppet government and German military. After they beat a team from a local German Air Force base, the league was disbanded and several of the team members arrested by the Gestapo and four were executed.
Apart from acting in the movie, Pelé also assisted in choreographing all the playing actions in the climactic game.
Reportedly, Sylvester Stallone insisted that his character score the game-winning goal in the film, as he felt he was the biggest star in the film. The non-American crew was finally able to convince him of the absurdity of the goalkeeper scoring the winning goal, and the penalty shot was specifically written to placate his ego.
The stadium used for the football match, although supposed to be the Stade Colombes in Paris, is actually in Budapest, Hungary. This is because the producers had difficulty finding a large stadium without floodlights, as floodlights at football stadiums were largely unknown until well after WW2. The MTK stadium in Budapest was the biggest one without lights (but at the same time structurally similar to Continental stadiums that were around during WW2) that they could find.
Sylvester Stallone initially received training from England's World Cup winning goalkeeper, Gordon Banks. After only a few days of training Stallone dismissed Banks, saying that anybody could play in goal without trouble and he didn't see what all the fuss was about. During filming however, Stallone constantly kept flinging himself about in an exaggerated acrobat-like manner and dislocated a shoulder, broke two ribs and received extensive bruises. Afterwards he admitted that he should have listened to Banks more carefully and commented that it was harder than fighting in the Rocky movies.
Other than Sylvester Stallone and Michael Caine the rest of the allied players (that play in the game) are actual soccer stars from various countries around the world, mostly from the 1970s and 80s. Some of them perform the "tricks" that they were famous for, such as Pelé with a bicycle kick.
Stallone lost about 40 lbs for the film because he didn't want a prisoner of war to look like an "Olympic Boxer" and he felt he needed that weight reduce to perform the tasks of a soccer goalie.
Stallone nixed the idea of using a professional player as a double for the game sequences, because of that he separated his shoulder and broke a finger.
John Wark had his Scottish accent dubbed to an English one.
A three acre prison set was built in the grounds of the Allag Riding Stables on the outskirts of Budapest.
250,000 Hungarian extras were drafted in as spectators of the football match.
One of the footballers, Mike Summerbee, became friendly with Michael Caine. After retiring from football, Summerbee went into bespoke shirt-making. Caine is one of his favored customers.