Sean Connery asked that his scenes be filmed quickly so he could get to Jamaica in time to star in Dr. No.
Sean Connery, who made his debut as James Bond also in 1962, acted in the movie along with Gert Fröbe and Curd Jürgens - two future Bond villains.
John Robinson, who plays Admiral Ramsay, actually took part in the D-Day landings.
Donald Houston who plays the one scene RAF Pilot actually was in the RAF during the Second World War.
Darryl F. Zanuck and Cornelius Ryan collaborated on the screenplay but they immediately hated each other upon their first meeting. It was up to producer Elmo Williams to liaise between the two and keep the peace.
Darryl F. Zanuck was continually at Andrew Marton's shoulder when he was directing the American sequences.
Darryl F. Zanuck was quoted in an interview as saying that he didn't think much of actors forming their own production companies, citing The Alamo, produced by John Wayne, as a failure of such ventures. Wayne found out about this interview before being approached by Zanuck, and refused to appear in the film unless he was paid $250,000 for his role (when the other famous actors were being paid $25,000). Wayne got his requested salary.
Alec Guinness was sought for a cameo.
Dewey Martin filmed scenes playing the cameo role of Lt.Wilder, but his scenes were deleted in post-production.
Richard Todd (playing Major John Howard, Officer Commanding D Company of The 2nd Battalion The Oxfordshire and Buckinghamshire Light Infantry, Air Landing Brigade, 6th Airborne Division) was himself in Normandy on D-Day, and participated as Capt. Todd of the 7th Parachute Battalion, 5th Parachute Brigade, British 6th Airborne Division. His battalion actually went into action as reinforcements, via a parachute jump (after the gliders had landed and completed the initial coup de main assault). Capt. Richard 'Sweeney' Todd was moved from the plane he was originally scheduled to jump from, to another. The original plane was shot down, killing everyone on board.
Richard Todd, veteran of the action at the bridge at Benouville (later renamed Pegasus Bridge) (see Item 1 above), was offered the chance to play himself but joked, "I don't think at this stage of my acting career I could accept a part 'that' small." He played the commander of the actual bridge assault itself, Major John Howard, instead.
Curd Jürgens plays General Blumentritt. In real life, Jurgens had actually been imprisoned by the Nazis.
Kenneth More, playing Capt. Colin Maud, carried the shillelagh Maud had used in the actual invasion. Maud loaned it to More so the actor could use it in the film.
Richard Dawson's film debut.
Leslie Phillips only has one line in this movie.
Eddie Albert, who played Colonel Thompson, was a World War II veteran. However, Albert actually served in the Pacific, not in Europe.
Russell Waters is credited by various who's who's as being in the film, but he is nowhere to be seen.
Henry Grace was not an actor when being cast as Dwight D. Eisenhower, but his remarkable resemblance to Eisenhower got him the role.
20th Century Fox were taking a real gamble making this film. At $10 million, it was a hugely daring venture, but even more risky was Cleopatra which was being filmed concurrently. This was to set Fox back the then unprecedented sum of $40 million. Although Cleopatra did well at the box office, it was simply too expensive to recoup its costs and nearly bankrupted the studio. Fortunately The Longest Day turned out to be one of their biggest hits to offset the damage caused by the Egyptian epic.