Charlie Hunnam was emaciated and had lost 20 pounds for the last season of the TV show Sons of Anarchy. During auditions, Guy Ritchie was very bothered by his look though he liked his performance and asked him 4 times during the process about his poor physique and 'What was the heaviest he had been'. Hunnam said that when Ritchie brought up the 4th time, he knew that the physicality of Arthur was very important to Ritchie. He promised Ritchie that he would get into incredible shape for Arthur and to prove his fitness for the part, offered to physically fight the other two finalists - Henry Cavill and Jai Courtney. Hunnam told Ritchie, "Look, dude, you keep bring this up, the physicality. Its 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 after this.

The first installment of a planned six films series.

Actress Katie McGrath who plays the role of Elsa in this film played Morgana, King Arthur's half-sister, in Merlin (2008) a BBC series based on Arthurian Legend.

The movie went through several stages. Firstly, 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.

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.

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

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."

Gary Oldman was cast as Merlin but he dropped out.

Charlie Hunnam has promised that "I will get into incredible shape for this film." He further added that, "I am going to do 500 push-ups everyday" saying he wanted to look like one of those guys who do that.

Astrid Berg├Ęs-Frisbey beat out Felicity Jones, Elizabeth Olsen and Alicia Vikander for the role of Guinevere.

Reunites Charlie Hunnam and Aiden Gillen from Queer as Folk.

Marion Cotillard was rumored for the role of Morgana.

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.

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 Guy Ritchie. The second round was a full-fledged audition. Ritchie instantly liked Hunnam after their 90 minute talk and told him, "I really fucking 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.

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."

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

It is the first installment of a planned six film series

Filming began on March 2, 2015.

Colin Farrell was offered a role but declined.

Both Aidan Gillen and Michael McElhatton are in the series Game of Thrones (2011) as well as Eline Powell.

Freddie Fox (Ed) has a sister Emilia Fox who portrayed Morgause, 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.

Katie McGrath (Elsa) and Georgina Campbell (Kay) have both worked in projects with actor Alfie Stewart. Katie worked with him in an episode of the TV series 'Merlin', as Daegel and Morgana respectively; Georgina worked with him in the TV series 'Sadie J', as Keith and Whitney respectively.

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

Charlie Hunnam said that he had to the gym everyday, even during filming, after shooting for 16 hours a day in order to maintain his "musclebound look" for the character. Hunnam put on a lot of muscle mass for the role but doing sword fighting for 16 hours day acted as cardio burning off his muscle. So he had to keep training throughout the shoot to maintain his muscle mass. He said towards the end, he had achieved the fitness level of a professional athlete due to the constant cardio work and gym work.

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

Mikael Persbrandt joined the film on March 6, 2015, to play a villainous role.

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 6% 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 26-pack abs (sic) ready to rumble."

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