Most of Sam J. Jones's dialog was dubbed. This was down to the fact that Jones had had a falling out with producer Dino De Laurentiis over lack of payment and refused to go into the recording studio to loop his lines.
This was the first movie that the rock band Queen made the music for, in it Prince Vultan says "Who wants to live forever?". The only other movie Queen did the soundtrack for was Highlander (1986), for the movie they wrote songs including "Who Wants to Live Forever" plus many others.
Kurt Russell auditioned to play Flash Gordon. According to an interview with Russell in Starlog magazine from August 1981, Dino De Laurentiis really wanted Russell for the part, but Russell ultimately turned it down because he thought the character was lacking in personality.
During the opening credits each actor's credit is accompanied by artwork of their character from the original comic strip. Therefore you see all of the major characters as cartoons before you see the actors who play them.
In the original script, when Flash is sentenced to death by Ming, Dale bursts out that Ming is "absolutely merciless". Ming is enthralled with the description, and immediately starts calling himself "Ming the Merciless".
Dr. Zarkov's backstory was that he was a NASA scientist who was fired for his paranoid fantasies that Earth was going to be attacked from outer space. Sixty Minutes derided him as "A Poor Man's Billy Mitchell".
Klytus and Kala, Ming's two chief henchmen, were competitors for their ruler's favor. Ming played them off against each other to keep them from teaming up against him. This was downplayed in the film to keep the storyline fluid.
If you pay close attention to the scene when Flash's luggage is being loaded onto the airplane at the beginning of the film, you'll see that one of the duffel bags is green and white and has the word "JETS" written on it. This is meant to indicate that Flash Gordon plays for the New York Jets football team. The team's colors are white and green and their logo resembles the wing of a jet.
The movie came about when producer Lou Scheimer, seeking additional funding for his animated NBC movie of the week Flash Gordon: The Greatest Adventure of All (1982), turned to Dino De Laurentiis, who agreed to help out if Scheimer could finagle the rights for him to make a theatrical film. Impressed with Scheimer's results and the prospect of cashing in on the theatrical version, NBC shelved the animated movie for a handful of years, had Scheimer's company recut it and turn it into the Saturday morning series Flash Gordon (1979).
In the original script, Flash and Dale first meet at a Canadian resort called Dark Harbor. Although they flirt with each other, they don't become acquainted until they're sharing the ill-fated plane ride to New York City. Dale later talks briefly about Dark Harbor during her tear-filled meeting with Flash before his execution.
Nicolas Roeg was originally going to direct, but didn't due to creative difference. One of his proposals was to excise the trademark cliffhangers and melodrama, seeing Flash as more of "a metaphysical messiah."