"You know when Homer Simpson doesn't know what to say, and the camera stays on him as his face is completely blank?", he asked. "That's what it was like when I first got offered this role by James (Gunn, director)".
The Peacemaker star has made the leap from DC to the MCU to play the villainous High Evolutionary in the third Guardians Of The Galaxy outing. Described by the star as a "villain with a God complex, born with a silver spoon in his mouth", the trilogy-closer delves deeper into the backstory of Rocket Raccoon, and how he was created and tortured by Marvel's mad scientist.
When he was first offered the role, Iwuji was behind enemy lines over at DC, having just finished filming the iconic opening credits sequence for Gunn's Suicide Squad spin-off series.
"We'd just finished doing a big dance sequence for Peacemaker, and James said, "Can I have a word with you? I have something I want to talk to you about."
"He started with, "I don't know what your schedule is, but I would like you to play the High Evolutionary in Guardians of the Galaxy Vol. 3". I just blinked for a while and mumbled something, I was in complete shock - I'm a jobbing actor, the closest I've got to Marvel is my local IMAX."
Within six weeks of the shoot, Gunn and Iwuji shot a screen test, and he was aboard the MCU. He then had to prepare himself to go to some mentally dark places to fully embrace the role of an evil genius who experiments upon living creatures, unsympathetic to how much pain they feel from the torture - and found his way into that mind space from an unlikely source of inspiration.
He continued: "The first thing that's written when you meet him is him listening to Space Opera, so immediately I knew classical music was going to be a big part of this character. Because I didn't know where I was going to go with him, I thought I would just listen to a lot of my favorite arias and classical compositions, and sent them to James; one or two of them actually made it onto the film.
"So, a lot of finding this guy, for me, was listening to opera - he's this mad scientist, and I thought of him as someone that doesn't sleep, who's obsessed with listening to this music. He needs the music as he wanders through his castle at night.
"And I thought very deeply about Henry IV. He has that great speech; "Heavy is the head that wears the crown. How many thousands of my poorest subjects that at this moment, are asleep," and I sent that to James.
"So, that's where the preparation wasn't specifically about answering "how do I play the character as he's written?", so much as it was about "how do I bring this guy into the room?" I knew that the character would be taken care of in the script, as his motivations are impeccably written there - it was entirely about finding who this guy really is before he turns up on the set."
Iwuji isn't the only actor joining the franchise in this final instalment, with Midsommar star Will Poulter another new addition to the ensemble as the more complicated villain Adam Warlock, a role he admits he was initially "scared of" due to how much he admired the previous films in the series.
He said: "You look at something as a fan, and you don't imagine yourself as being a part of it, so that in itself was exciting. As a new kid, I was scared of joining this franchise, but everybody involved in it made it pretty easy."
Being a newcomer to the cast, unfamiliar with the group dynamics on set going into the job, proved surprisingly beneficial to Poulter when it came to getting into character.
"I think the thing I shared the most with Adam Warlock was probably just that sense of trying to work out what the hell was going on", he continued. "When you find a parallel between your character and your own experience, it can be something to latch on to.
"I was the new kid on the block arriving on this set. Very much like how Adam Warlock was arriving into the world, trying to find his way and work out the rules."
His casting as a famous comics villain - and his subsequent beefing-up to get into character - set the internet alight. However, there was one group of people who initially weren't impressed that he bagged the role: his family.
"It's funny, my family are mostly medics, nurses, and doctors", he laughed. "It sounds cheesy, but they are real-life superheroes - I'm a huge disappointment compared to them!
"They're very supportive and very excited for me, obviously, but my mum and dad were the only people within my family and group of friends to have not seen the Guardians films. They've watched them all back-to-back since I got the role, so a lot has changed since then."
We certainly hope that Will's parents aren't left disappointed by this epic conclusion.
Guardians Of The Galaxy Vol. 3 is out now in UK cinemas.Read what James Gunn and Chris Pratt have to say about it here.
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