Helen Mirren and Nicol Williamson were initially reluctant to work with each other, as they had both been in a disastrous production of "Macbeth" and were not on speaking terms. John Boorman cast them because their natural animosity would be perfect. According to Mirren, she and Williamson, free from the problems with Macbeth, "wound up becoming very good friends" during filming.
The initial fight scene in the movie had to be filmed three times. It was filmed at night and the first two times, all the film came out underexposed due to a fault in the exposure meter. The cameraman had a nervous breakdown over the issue and quit.
The names of the 25 knights inscribed on the Winchester Round Table are given as: Galahad, Lancelot du Lac, Gawain, Percivale, Lionell, Bors de Ganis, Kay, Tristram de Lyones, Gareth, Bedivere, Bleoberis, La Cote Male Taile, Lucan, Palomedes, Lamorak, Safer, Pelleas, Hector de Maris, Dagonet, Degore, Brunor le Noir, Le Bel Desconneu, Alymere, and Mordred.
Before the final battle, Arthur's knights are camped around a "Stonehenge" like formation on a hill. The rocks were fake and part of the set. According to John Boorman, some American tourists were driving by down below and saw the formation. Thinking they were real, the tourists hiked up the hill, and Boorman had to explain to them that they were not real, but part of a movie set.
All of the armor used in the film was hand made out of aluminum, primarily by British armorer Terry English. English custom-fit the suits of armor for the principal characters, but kept the overall style the same for members of different groups, especially prominent in the nearly "uniform" armor of the Knights of the Round Table. English can be seen in the film during the tourney scenes; he is the blacksmith who looks up at Arthur (Nigel Terry) when the future king is chasing a thief and stops to contemplate filching a sword from the armorers' tent.
The original cut of the film was over three hours long. Among the many scenes that were lost - but was briefly glimpsed in the trailer - was a scene where Lancelot rescued Guenevere from a forest bandit.
The Charm of Making spoken by Merlin and Morgana, is an attempt at Old Irish that translates to: "Serpent's breath, charm of death and life, thy omen of making." The phonetic rendering, as spoken in the movie, is: /ana:l nathrakh, u:rth va:s bethud, dokhje:l djenve:/. In Irish, the phrase is: 'Anáil nathrach, ortha bháis bheatha, do thuar dhéanamh', which is pronounced similarly, but not exactly as in the movie.
John Boorman had wanted to call this "Merlin", but CBS had a show called Mr. Merlin (1981), so they had exclusive usage of the "Merlin" name. It began shooting as "Knights", but Ridley Scott had registered "Knight" for a medieval epic, he was planning with Walter Hill, as his follow-up to Alien (1979). They were already shooting before they decided on "Excalibur".
John Boorman had planned a film adaptation of the Merlin legend, as early as 1969, but the studio, to which he offered it (United Artists), rejected his concept, offering him The Lord of the Rings instead. When that fell through, he went back to this film.
Nigel Terry was 35 when playing Arthur from a teenager to a 20-to-30-something king, and then finally as an old man. Similarly, 29-year old Cherie Lunghi played Guinevere from teenager to full adult queen to an old woman living out the rest of her life in quiet misery as a nun.
The fight between Arthur and Lancelot was filmed on the Powerscourt estate. The battle scenes in Sir Laurence Olivier's film version of Henry V (1944) were also filmed there due to wartime restrictions in England.
The graphic scene, in which Arthur is impaled with Mordred's spear, and then pulls himself towards Mordred, was mirrored in Star Trek: Nemesis (2002), when Shinzon, who is impaled with a pole, pulls himself toward Captain Picard. Sir Patrick Stewart plays Leondegrance.
During the making of The Tailor of Panama (2001), Pierce Brosnan told John Boorman that his "Excalibur" audition had been a disaster. After it ended, he couldn't get his battered old car to start, and that he had to abandon it, and walk home on a dismally wet day. When he got back to his small bedsit flat, he felt that the day had gone so badly, that he had considered quitting acting altogether.