Dame 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. Writer, Producer, and Director 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 three times. It was filmed at night, and all of the film came out underexposed the first two times, due to a fault in the exposure meter. The cameraman had a nervous breakdown over the issue and quit.
The names of the twenty-five 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 Writer, Producer, and Director 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.
The original cut of this movie was over three hours long. Amongst the many scenes that were lost, but was briefly glimpsed in the trailer, was a scene where Sir Lancelot rescued Guenevere from a forest bandit.
All of the armor used in the movie 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 movie during the tournament 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 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.
During the making of The Tailor of Panama (2001), Pierce Brosnan told Writer, Producer, and Director 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.
John Boorman had planned a movie 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 movie.
The fight between King Arthur and Sir Lancelot was filmed on the Powerscourt estate. The battle scenes in Sir Laurence Olivier's Henry V (1944) were also filmed there due to wartime restrictions in England.
Nigel Terry was thirty-five when playing Arthur from a teenager to a twenty-to-thirty-something King, and then finally as an old man. Similarly, twenty-nine-year old Cherie Lunghi played Guenevere from teenager to full adult Queen to an old woman living out the rest of her life in quiet misery as a nun.
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".
In the final battle scene before King Arthur (Nigel Terry) is saved by Sir Lancelot (Nicholas Clay), one of the other knights rears back to strike a foe and hits King Arthur 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.
Zack Snyder, who is a fan of this movie, referenced this movie in Batman v Superman: Dawn of Justice (2016), which, in the opening credits sequence, this movie is on the marquee at the theater which young Bruce Wayne saw his parents murdered by a mugger.
Almost 20 years after the film's release, Liam Neeson starred as Jedi master Qui-Gon Jinn in Star Wars: Episode I - The Phantom Menace (1999). George Lucas, the writer and director and creator of the Star Wars film franchise had cited the legend of King Arthur as one of his influences behind Star Wars.
Liam Neeson, Dame Helen Mirren, and Ciarán Hinds appeared in adaptations of DC Comics properties: Liam Neeson in Batman Begins (2005) and The Dark Knight Rises (2012), Dame Helen Mirren in Red (2010) and Red 2 (2013), and Ciarán Hinds in Justice League (2017). Sir Patrick Stewart played Professor Charles Xavier in the X-Men film franchise, based on the comics created by Marvel Comics.
This movie has a Star Wars connection: Creator George Lucas had cited the legend of King Arthur as one of his inspirations behind Star Wars. Charley Boorman's best friend Ewan McGregor played young Obi-wan Kenobi in Star Wars: Episode I - The Phantom Menace (1999), Star Wars: Episode II: Attack of the Clones (2002), and Star Wars: Episode III: Revenge of the Sith (2005). Liam Neeson played Qui-Gon Jinn in Star Wars: Episode I - The Phantom Menace (1999), and Keira Knightley, who played Sabe in Star Wars: Episode I - The Phantom Menace (1999), played Guenevere in King Arthur (2004).
Nigel Terry and Clive Swift later appeared opposite David Tennant in the revived series of Doctor Who (2005) (TV Series). In the final season of the classic series of Doctor Who (1963) (TV Series), The Doctor (Sylvester McCoy) and his companion Ace (Sophie Aldred) encountered Morgaine and Mordred whom come from another dimension and have come to their dimension to seek possession of King Arthur's sword Excalibur which they mistake The Doctor for Merlin and The Doctor and Ace discover King Arthur's body and Excalibur in a submerged spacecraft at the bottom of a lake.
Liam Neeson and Ciarán Hinds played villains in movies featuring Bruce Wayne (Batman): Liam Neeson played Ra's Al Ghul in Batman Begins (2005) and The Dark Knight Rises (2012), and Ciarán Hinds played Steppenwolf in Justice League (2017).
The trivia item below may give away important plot points.
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.