"The American Legion Zouaves of Richard F. Smith Post No. 29, Jackson, Michigan" were a U.S. Civil War reenactment group. They performed the intricate high speed marching maneuvers during the knighting ceremony. The United States Army adopted the Model 1863 Zouave rifle, a percussion or "cap-and-ball" muzzle-loader, which was manufactured by Remington. Obviously the marching knights could not be armed with Civil War-era rifles in the movie. The original Zouave units were North African regiments of the French Army, beginning earlier in the 1800s and serving through both World Wars.
Basil Rathbone was a world-class fencer and it was due to his efforts that the hilarious fencing scene was filmed without injury. He later admitted that several times he was almost skewered by Danny Kaye's sword.
Danny Kaye's daughter, Dena Kaye, said for the rest of his life, when people recognized Danny in a restaurant, they would walk up and spout the entire "brew that is true" speech.
In the famous "snapping" swordfight between Danny Kaye and Basil Rathbone, Kaye's sword movements were too fast for Rathbone, who was 63 at the time. The film's fight choreographer dressed up as Rathbone's character and was filmed from behind for the fast sections. If you look, you can see that most of the fight consists of "Rathbone" from the back, then shots of the real Rathbone standing "en garde".
Some songs that were written but not heard in the film are "I Live To Love" (sung by Danny Kaye to Angela Lansbury when he swings into her bedroom) and an extended "Pass the Basket" number when Kaye appears before the King (just prior to the famous "Maladjusted Jester"). Both songs were, however, recorded and released on the film's companion record.
The "Now I can shoot and toot" speech during "The Maladjusted Jester" was previously said by Danny Kaye in his first feature role in Up in Arms
Unimpressed with him in tights, producers of the film made Danny Kaye wear 'leg falsies' to improve the shape of his legs. This adds a touch of irony when Hubert Hawkins offers the princess all of him, including his legs and calves.