All-Star Cast: Patrick Stewart (Leondegrance), Liam Neeson (Gawain), Gabriel Byrne (Uther), Helen Mirren (Morgana), Nicol Williamson (Merlin), and Ciarán Hinds (Lot). This is actually an inversion, as none of them were especially big at the time, at least not in the United States, and John Boorman chose them because they were relatively unknown and didn't want big stars to overshadow the production. Which makes this Hilarious in Hindsight.
California Doubling: The movie was shot entirely in Ireland, and was a major boon to the country's film industry.
Darkhorse Casting: John Boorman wanted the movie to focus on the story, not the actors and actresses, so he cast relative unknowns. At the time, only Nicol Williamson was relatively familiar to American moviegoers.
Dawson Casting: Nigel Terry was 35 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.
Deleted Role: Peter Benson had a featured role that was removed in post-production.
Divorced Installment: John Boorman originally wanted to do a film adaptation of J.R.R. Tolkien's The Lord of the Rings trilogy, compressing the trilogy into one very long film, but he couldn't get the rights to it so he made Excalibur instead. Reportedly his screen treatments sexed up Tolkien's story quite a bit, including but not limited to giving Frodo Baggins nude scenes with Arwen and Galadriel and having the House of Healing scene between Eowyn and Aragorn feel like a sex scene (as he planned to have Eowyn settle down with Aragorn, which reportedly was Tolkien's original idea). Generally the Arthurian myth sounds like it was better suited to him than Tolkien!
Fake Nationality: Patrick Stewart as a French Kingnote King Leodegrance, whose kingdom is Cameliard (you can hear him shouting it a number of times during the tournament scene). While some scholars do think Cameliard was meant to be in Brittany, most think it's meant to be located in Cornwall.. And not for the last time.
The equally English Nicholas Clay as the equally French Lancelot du Lac.
Friendship on the Set: Nicol Williamson and Helen Mirren had previously co-starred in a disastrous production of Macbeth where they absolutely hated each other. John Boorman felt that their natural animosity would be perfect for their roles. Neither realised the other would be in the film until filming started. Mirren recalled that they got on very well once they were "freed from the shackles of Macbeth".
John Boorman used many of the same locations that he had used for Zardoz. In fact, most of these are quite close to his house in Annamoe, County Wicklow, Ireland.
The fight between King Arthur and Sir Lancelot was filmed on the Powerscourt estate. The battle scenes in Henry V (1944) were also filmed there due to wartime restrictions in England.
Hostility on the Set: The Irish extras fighting each other did not want to stop after Boorman yelled cut, "to settle old scores".
Production Nickname: This movie was referred to as The Boorman Family Project.
Reality Subtext: The animosity between Merlin and Morgaine wasn't just acting. Nicol Williamson and Helen Mirren had previously co-starred in a disastrous production of Macbeth, where they couldn't stand each other. John Boorman cast them because he thought their natural animosity would be perfect. Neither realised the other would be in the film until filming started.
Real-Life Relative: Queen Igrayne and her grandson Mordred (as a boy) were played by John Boorman's daughter Katrine and son Charley. Another daughter, Telsche, played the Lady of the Lake.
It might count as Fridge Horror when you realize that, in casting his daughter Katrine to play Igraine, Boorman directed his own daughter in a pretty aggressive sex scene.
The film is one of Zack Snyder's favourite films, so much so that a number of his own films have overt references to it. In 300, one of the Spartans goes down similarly to Arthur vs. Mordred, and this is repeated in Batman v Superman: Dawn of Justice, with Superman vs. Doomsday, though here he's closer to the myth by having Superman use the spear, as Boorman had flipped the weapons around.
The first two words of the Charm of Making are "Anaal Nathrakh"
When Uryens knights Arthur with Excalibur, he's visibly trembling from emotion. In reality, Keith Buckley was shivering from the cold; they were in the castle moat.
Baby Arthur grabbing a hold of Igrayne's hair was a coincidence.
Sean Connery was attached to play King Arthur in the early stages, and was also considered for Merlin. He would go on to play the former in First Knight.
Working Title: John Boorman had wanted to call this Merlin but CBS had a show called Mr Merlin 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. They were already shooting before they decided on Excalibur.