Helen Mirren and Nicol Williamson were initially reluctant to work with each other. They'd 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 "wound up becoming very good friends" during filming.
The initial fight scene had to be filmed 3 times. It was filmed at night, and all of the film came out underexposed the first 2 times 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: 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 aluminium, 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 planned a film adaptation of the Merlin legend as early as 1969. United Artists rejected his concept, offering him The Lord of the Rings instead. When that fell through, he went back to this film.
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.
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".
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.
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 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.
Liam Neeson, Helen Mirren and Ciaran Hinds would all later star in adaptations of DC Comics properties: Liam Neeson in Batman Begins (2005), Helen Mirren in Red (2010) and Red 2 (2013) and Ciaran Hinds in Justice League (2017). Patrick Stewart would play Charles Xavier/Professor X in the X-Men films, based on the comics created by DC's rival publication, Marvel Comics.
Liam Neeson and Ciaran Hinds would both play villains in films featuring Bruce Wayne/Batman: Liam Neeson would play Ra's Al Ghul in Batman Begins (2005) and Ciaran Hinds would play Steppenwolf in Justice League (2017).
In the final battle scene before Arthur is saved by Lancelot, one of the other knights rears back-to strike a foe and hits Arthur (Nigel Terry) on the back of the head with his sword. You can see it caused pain to Terry, who grabbed the back of his head, but continued fighting.