Charlie Hunnam had lost 20 pounds for the last season of Sons of Anarchy (2008). During auditions, director Guy Ritchie was very bothered by his look, though he liked his performance, and asked him four times during the process about his poor physique. Hunnam said that when Ritchie brought up the fourth time, he told Ritchie, "Look, dude, you keep bringing this up, the physicality. It's obviously your primary concern. So if you want to do away with all this auditioning bollocks, I'll fucking fight those other two dudes. I know who they are. You can bring them both in here. I'll fight them both. The one who walks out the door gets the job." Hunnam won the role.
Charlie Hunnam promised that he would get into incredible shape for this film, even saying he was going to do 500 push-ups everyday, as he wanted to "look like one of those guys who do that." After filming concluded, Hunnam revealed that he did in fact do several hundred push-ups every day, with his personal record being 1,050 push-ups in a single day.
Guy Ritchie said he was shocked by how shredded Charlie Hunnam got for this role and was horrified (in a good way) when Hunnam took off his shirt for the first time. He marveled, "Charlie never seems to carry more than six percent body fat. When he first took his top off, I was horrified. Usually with actors, they take their top off and I'm horrified in the wrong way. But Charlie walked in all pleased with himself with his twenty-six-pack abs (sic) ready to rumble."
Charlie Hunnam said that he closely observed Ultimate Fighting Championship Irish featherweight Conor McGregor to incorporate his fighting style and his ultra-ripped physique into his portrayal of a young "street" version of Arthur. Hunnam watched hours and hours of interviews and fight footage of McGregor on YouTube because he thought he would be an interesting model for the character.
The film went through several stages. First, director David Dobkin tried to make a version with Kit Harington as King Arthur and Joel Kinnaman as Lancelot, but the movie was deemed too expensive and the actors too unknown. Then, Colin Farrell was cast as King Arthur and Gary Oldman as Merlin, but that version wasn't made either.
Henry Cavill and Jai Courtney were the other two finalists for the role of King Arthur, apart from Charlie Hunnam. The final audition comprised two rounds. The first was a sit-down chat with director Guy Ritchie and the second round was a full-fledged audition. Ritchie instantly liked Hunnam after their ninety-minute talk and told him, "I really f***ing dig you, bro. I hope you act as well as you talk, and I look forward to seeing you tomorrow." Hunnam performed equally well in the audition and won the role.
The film was pitched to the studio and the actors as Lord of the Rings meets Guy Ritchie's own Snatch (2000). Charlie Hunnam said that description sold him on the movie, saying, "That's a film I wanted to see."
Guinevere was originally going to be a part of this movie, as Arthur's love interest. However, Guy Ritchie said he couldn't give that story line enough screen time, so the character was rewritten and became Mage.
Charlie Hunnam's introduction scene in the film was originally very different. The finished film introduces him shirtless, doing some intense wrestling and shadow-boxing (punching the air), but Hunnam revealed it was a much bigger sequence earlier, saying, "We had this whole montage planned out with pull-ups and push-ups and heavy bag boxing -- a proper 'Rocky' montage. And the shadow boxing that I do was just me trying to stay in the zone in the takes, and that's ultimately what Guy chose to use in the film."
Idris Elba was reportedly strongly considered for the role of Merlin in the film, but it turned out to be just a rumor as Elba commented on the matter, stating, "Unfortunately, I'm not in the 'King Arthur' movies. That was a rumor. That wasn't true, unfortunately. But I wish the guys a lot of success."
Sam Lee recorded a new version of "The Wild Wild Berry" specifically for the trailer. It was initially only meant to be for the trailer, but due to overwhelming fan demand, a full version was recorded and released on the soundtrack.
The song that plays during the latter part of the ComiCon trailer is an adaptation of the traditional song "Wild Wild Berry," which is itself an adaptation of the traditional song "Lord Randall." In both cases, the lyrics speak of a young lord who comes home from feasting and wants nothing but to lay on his bed and rest. When questioned, he reveals he was poisoned by the woman he took to be his true love. After the lord's death, the woman who poisoned him is executed.
A common mistake is that Arthur was a legendary king of England. He was in fact a legendary king of Britain or Britannia to be exact. The first Arthurian legends were Welsh. The Welsh (and Cornish) are the direct descendents of the ancient Britons who inhabited the island of Britain until the coming of the Saxons pushed them to the western edges of the island and south to Brittany (hence the name and the fact Arthur is also a Breton folk hero). Although the Anglo Saxons who built England probably knew the Arthurian ballads it wasn't until after the Norman invasion that the (Christianised) tales became popular in England once again.
The opening of the film features a sequence in which King Uther rides out to personally repel the invading army. The musical score for this sequence is actually a piece of music composed by Gareth Williams from 2009. It was created for 'The Life', a live-action advertisement for the video game Halo 3: ODST (2009). Daniel Pemberton (lead composer of this film) confirmed that director Guy Ritchie always intended to use this piece of music in Uther's opening sequence, as Ritchie is a self-professed gamer and Halo fan and had always wanted to use that piece of music in one of his films. Thus, Ritchie and Warner Bros. licensed the music (itself called 'The Life') from Microsoft for use in this film and inclusion on the film's soundtrack (in the track titled "The Politics and The Life"). Interestingly, despite years of interest from Halo fans, the release of the soundtrack for this film marks the first time that Williams' piece of music has been made available for purchase or download in any form.
Sticky Studios developed the official "King Arthur: Legend of the Sword" game for iOS and Android. It uses the "dojo" area as seen in the trailer as a setup to show Arthur's street fighting skills. The game uses swipe based gestures for hand combat and wielding the Excalibur sword. The studio also designed a virtual reality activity for Google Daydream, which allows players to fight using motion gestures.
Freddie Fox (Ed) has a sister, Emilia Fox, who portrayed Morgause, the maternal half-sister of Morgana (King Arthur was Morgana's paternal half-brother), which was portrayed by Katie McGrath (Elsa in this film), in Merlin (2008), a BBC series based on Arthurian Legend.
Aidan Gillen (Bill), Michael McElhatton (Jack's Eye), and Eline Powell (Syren 2) have all starred on Game of Thrones (2011). Gillen stars as Petyr 'Littlefinger' Baelish, while McElhatton starred as Roose Bolton, and Powell as Bianca. Only Gillen and McElhatton shared scenes together in the T.V. series.
The trivia items below may give away important plot points.
Unusual for a King Arthur film, neither Merlin nor Lancelot are featured (though the former is mentioned repeatedly), due to the intention of saving them for the planned sequel. For similar reasons, all references to the Mage actually being Guinevere were removed during post-production.
The shot which establishes the dark warrior who kills Arthur's mother and attempts to take Excalibur from Uther seems heavily inspired by Frank Frazetta's own Dark Warrior painting: same angle, intention, and pretty much the same overall design.