This is the first installment of a planned six-film series.

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 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.

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."

Felicity Jones, Elizabeth Olsen, and Alicia Vikander auditioned for the role of the Mage.

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.

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."

Originally, Joel Kinnaman was cast as Lancelot and Kit Harington was cast as King Arthur. The movie was delayed because the studio wanted a more well-known cast, with a greatly reduced budget.

Actress Katie McGrath, who played the role of Elsa in this film, also played Morgana, King Arthur's half-sister, in Merlin (2008).

Director Guy Ritchie's first cut reputedly came in at three hours and thirty minutes.

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.

The film's original title was "Knights of the Roundtable: King Arthur."

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."

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.

Charlie Hunnam said that he achieved his biggest size ever for King Arthur (2017). He stated, "I got big -- bigger than I've ever been. I got to 190 pounds!"

Mikael Persbrandt dropped out of the long-running "Beck" series, which made him a star in Sweden, to appear in this 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."

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.

This is the fourth film based on Arthurian legend to be produced and distributed by Warner Bros. Pictures, following Camelot (1967), Excalibur (1981), and The Magic Sword: Quest for Camelot (1998).

Asa six-year-old child, Charlie Hunnam was so obsessed with director John Boorman's Arthurian legend Excalibur (1981), he watched it repeatedly over the years.

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.

Maciej Kwiatkowski, Wojciech Roguski, and Slawomir Kurek from Prime Fury Stunt Team, which did all motion captures for The Witcher 3: Wild Hunt (2015) game, did the motion capture stunts, performance, and fight choreography.

Very little of Joby Harold's script was used. Producer Lionel Wigram rewrote it considerably.

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.

On January 21, 2016, Warner Bros. announced this film would be pushed back from February 17, 2017 to March 24, 2017. The title was also changed to "King Arthur: Legend of the Sword" in July 2016.

Warner Bros. had also tried to remake John Boorman's Excalibur (1981), with Bryan Singer directing. When Jack the Giant Slayer (2013) failed at the box office, the remake was cancelled.

This film reunited Charlie Hunnam and Aidan Gillen from the UK version Queer as Folk (1999) in which they played lovers.

Marion Cotillard was rumored for the role of Morgana.

The release date was delayed over ten months due to filming delays.

The name of David Beckham's character, Trigger, is a reference to the character of the same name from Only Fools and Horses.... (1981). Trigger was intended to appear alongside Beckham in the charity special Only Fools and Horses: Beckham in Peckham (2014), but was written out due to the illness and subsequent death of actor Roger Lloyd Pack.

Charlie Hunnam and Jude Law previously starred together in Cold Mountain (2003). Law played the protagonist, while Hunnam played the antagonist.

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.

Filming began on March 2, 2015.

This film marks the third collaboration between Jude Law and director Guy Ritchie. They previously worked together on Sherlock Holmes (2009) and Sherlock Holmes: A Game of Shadows (2011).

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.

Charlie Hunnam and Eric Bana previously starred together in Deadfall (2012).

This film reunited Charlie Hunnam and Geoff Bell, both of whom were in Green Street (2005)_.

Katie McGrath (Elsa) and Georgina Campbell (Kay) have both worked in projects with actor Alfie Stewart. McGrath worked with him in Merlin: The Hollow Queen (2012), as Daegel and Morgana, respectively, and Campbell worked with him in Sadie J (2011), as Keith and Whitney, respectively.

In this film, Charlie Hunnam and Jude Law trade places as the main protagonist and his nemesis from the movie Cold Mountain (2003).

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.

This film received a "B+" CinemaScore [Variety 2017].

Eline Powell, who portrays one of the Three Syrens, is also the star of the mermaid drama "Siren" (2018).

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.

The name of the female Mage is never revealed, but her role in the plot (as Lady of the Lake and keeper of Excalibur) is that of Nimue in traditional Arthurian legends.