* ActorSharedBackground: UsefulNotes/WorldWarII ended less than 20 years before this movie came out, a number of cast members were actual veterans.
** Richard Todd, who plays Major Howard of the British Airborne, was a British paratrooper during the war and actually took part in the D-Day landings -- in fact, he was initially approached to play ''himself'' in the film.
** As a 22-year-old private, Joseph Lowe landed on Omaha Beach on D-Day with the Second Ranger Battalion and scaled the cliffs at Point-Du-Hoc. He scaled those 100-foot cliffs all over again, for the cameras, some 17 years later.
** Donald Houston, who has one scene as an RAF pilot, actually was in the RAF during World War II.
** Eddie Albert, who played Colonel Thompson, was a World War II veteran, but he served in the Pacific, not in Europe.
** Creator/HansChristianBlech (Werner Pluskat) was a German army veteran, although he fought on the Eastern front.
* AllStarCast: Paul Anka, Creator/RichardBurton, Red Buttons, Creator/SeanConnery [[note]]retroactively a star, those were his pre-James Bond days, and the role was pretty small[[/note]], Mel Ferrer, Creator/HenryFonda, Leo Genn, Jeffrey Hunter, Alexander Knox, Creator/RoddyMcDowall, Sal Mineo, Creator/RobertMitchum, Creator/RobertRyan, George Segal, Rod Steiger, Richard Todd (actually playing the Captain he served under on D-Day), Robert Wagner (again, pre-fame) and Creator/JohnWayne, in addition to a number of French and German actors who were already household names in their own countries such as Creator/{{Bourvil}}, Creator/CurdJurgens, Creator/HansChristianBlech and Creator/GertFrobe.
* BackedByThePentagon: About 23.000(!) real US, British and French soldiers were used as extras in the movie. They also rented two Messerschmitts from the Spanish air force and four Spitfires from the Belgian air force.
* TheDanza:
** Sort of. Gen. James Gavin, played by Robert Ryan, was actually born James Ryan, but put up for adoption at age two, and adopted by Martin and Mary Gavin at age seven.
** Creator/RichardBurton plays Richard Campbell.
* DawsonCasting:
** Creator/JohnWayne as Lieutenant Colonel Benjamin Vandervoort. The real Benjamin Vandervoort was 12 years younger than Wayne, and on D-Day he had been just 27, while Wayne was '''54''' when the movie was filmed.
** At 52, Robert Ryan was 15 years older than General James M. Gavin had been.
** Creator/RichardBurton said he felt that both he and Donald Houston were too old to play RAF pilots. During his national service in the RAF he never saw a pilot older than 30.
* DeletedRole: Dewey Martin, whose character was named Lt. Wilder, was deleted from the final cut.
* FakeIrish: Creator/SeanConnery has a small part as Private Flanagan.
* HostilityOnTheSet:
** Creator/JohnWayne (a very conservative Republican) and Robert Ryan (a very liberal Democrat) had managed to put their political differences aside when they made ''Film/FlyingLeathernecks'', but they did not get along at all while making this film.
** Darryl F. Zanuck and Cornelius Ryan collaborated on the screenplay, even though they hated each other almost from the first time they met. It was up to producer Elmo Williams to mediate between the two and keep the peace.
* MoneyDearBoy: This was one of several high-profile projects which Creator/JohnWayne took in the wake of the extremely expensive ''Film/{{The Alamo|1960}}''. He had used his own funds to help finance the project, and he was in desperate need of a quick payday.
* PlayingAgainstType: Red Buttons usually played goofy comic-relief or funny hero types. Here, while there's some humor with him, his main scene is one of the film's most horrific parts.
* ScrewedByTheNetwork: Averted - Fox was planning on just doing a quick wide release to earn some cash they needed to offset their pre-release losses from ''Film/{{Cleopatra}}''. Darryl Zanuck threatened to bring''The Longest Day'' to Warner Bros. if they didn't do a proper roadshow release, and Fox had to comply. (it still was the box office hit the studio needed)
* ThrowItIn: While clearing a section of the Normandy beach near Pointe du Hoc, the film crew found a tank used in the invasion. They cleaned it up and used it for the British Invasion scenes.
* WagTheDirector: As part of Creator/JohnWayne's contract, in addition to his high fee, he insisted on getting separate billing. The usual practice in film credits for this type of situation is to start off with "Starring John Wayne and ''the other actors''; however, the credits begin with "starring ''the other actors''... and John Wayne". Wayne's name appears last on the credits, while still meeting the separate billing clause of his contract.
* WhatCouldHaveBeen:
** Creator/ChristopherLee, who fought in the War as an intelligence officer with the RAF, auditioned for any role he could get. He was turned down because the producers felt [[{{Irony}} he didn't look like a military officer]].
** This was made just after UsefulNotes/DwightDEisenhower's presidency and the producers very nearly '''cast Eisenhower to play himself'''. They abandoned the idea because he was too old, though.
** Creator/JohnWayne's role was originally offered to Creator/CharltonHeston. Creator/WilliamHolden was offered the role, but he turned it down, as he was exhausted after finishing ''Satan Never Sleeps'', ''The Counterfeit Traitor'' and ''The Lion''.
** Creator/AlecGuinness was sought for a cameo.
** The part of the British padre was first offered to Dirk Bogarde, who turned it down.
** Creator/BrigitteBardot and Marina Vlady turned down the role of Janine Boitard.
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