For the scenes where Steven and Marc interact with each other, rather than have Oscar Isaac try to act against a green screen stand-in or an extra, Marvel hired Isaac's brother, the journalist-actor Michael Benjamin "Bro Dameron" Hernandez, to serve as his double as a way to help Isaac act opposite himself. Isaac said during a 'Moon Knight' press conference that his brother is the closest thing to him there is on earth. This way, Isaac was able to seamlessly differentiate between the characters and was able to feed off of Hernandez's energy, as if he was acting against himself.
Oscar Isaac told Empire magazine in February 2022 that he invited his neighbor, Ethan Hawke, to play the role of Arthur Harrow in 'Moon Knight' after bumping into him at a coffee shop in Brooklyn during the COVID-19 pandemic and after watching Hawke in The Good Lord Bird (2020). Isaac is also an executive producer on the series.
Since Marvel Studios decided to set "Moon Knight" in London to differentiate it from the many other titles set in New York City, Oscar Isaac decided to give Steven Grant a high-pitched, British accent, even though that wasn't indicated in the script. Isaac's British accent was criticized by the audience as soon as the first trailer was released. Isaac explained that he put on a London English accent for Steven that he suggested was intentionally bizarre and unconvincing. He was inspired by the accents of the Jewish community living in Enfield, London, as well as English comedians such as Karl Pilkington and Peter Sellers.
Arthur Harrow, the main villain in the show, was a minor character who appeared in just one issue of the "Moon Knight" comics, the second issue of Volume 2, which was published in April 1985. The show's creator Jeremy Slater said during a Q&A in his Twitter account on March 29, 2022, that Arthur Harrow was chosen as the main villain for the show because Moon Knight's only recognizable villain was Bushman, who just felt too close to Black Panther's Erik Killmonger, so they decided to create a new villain instead. Slater also said that Ethan Hawke in particular was instrumental in creating Harrow.
The QR codes shown throughout the series really work and when scanned by a smartphone, they redirect to a page on the official Marvel website where it's possible to read a free online issue of the Moon Knight comics related to the latest episode.