Even more unexpectedly, he got the role after a chance encounter with actor Oscar Isaac in a New York coffee shop.
The Training Day and Boyhood star Ethan explained to the Hollywood Reporter how he bumped into Oscar at their neighbourhood café and soon bonded over various career parallels.
Star Wars: The Last Jedi and Dune star Oscar was already signed up to make Moon Knight, playing the title role, and he encouraged Ethan to abandon his superhero aversion and sign up as the cult leader Arthur Harrow.
The duo also realised that they have another mutual connection – a love of jogging.
“We both realised that we like to run. So it was a great way to talk and get to know each other and make small talk about the show and work out problems with our scripts,” said Ethan.
“That’ll probably be what I remember most about our summer. Just battling out the scripts as we ran around Budapest.”
Marvel’s Moon Knight is a standalone series that sits inside the Marvel Cinematic Universe but is not connected to the other MCU programmes and stars Oscar as a mercenary, Marc Spector, who suffers from dissociative identity disorder, meaning he can can transition between different personality states without being aware of it.
Ethan, meanwhile, plays religious leader Harrow, who sees the Moon Knight as an impediment to him healing the world.
Ethan says the experience of making the series was empowering rather than daunting, as he’d feared.
“[Marvel Studios has] tremendous faith and belief in the actor’s ability to contribute. That opening scene is a great example of [Marvel] using your creativity to help get you to invest in the show and to come up with original. So I was really impressed by that.
“At one point when Oscar and I were rehearsing, I said to him, ‘You realise that they’re being so kind? They believe in us so much that if this doesn’t work, it’s our fault. We can’t blame anybody.’ But that’s really empowering as a performer. Yes, you have to work in their kitchen, but they’re going to let you work.”
Moon Knight will run for six episodes on Disney Plus from Wednesday, Mar 30.
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Patrick McLennan is a London-based journalist and documentary maker who has worked as a writer, sub-editor, digital editor and TV producer in the UK and New Zealand. His CV includes spells as a news producer at the BBC and TVNZ, as well as web editor for Time Inc UK. He has produced TV news and entertainment features on personalities as diverse as Nick Cave, Tom Hardy, Clive James, Jodie Marsh and Kevin Bacon and he co-produced and directed The Ponds, which has screened in UK cinemas, BBC Four and is currently available on Netflix.
An entertainment writer with a diverse taste in TV and film, he lists Seinfeld, The Sopranos, The Chase, The Thick of It and Detectorists among his favourite shows, but steers well clear of most sci-fi.