Why Robert Downey Jr. Chose Oppenheimer for His Post-MCU Debut

In Christopher Nolan's Oppenheimer (hitting the big screen July 21), Robert Downey Jr. plays the role of Lewis Strauss, who could almost be described as the anti-Iron Man.

A leading member of the Atomic Energy Commission, Strauss was among those responsible for stripping J. Robert Oppenheimer of his top-level government security clearance once the Manhattan Project physicist began publicly voicing serious doubts over the newly-created atomic bomb and its even more destructive progeny: the thermonuclear hydrogen bomb. Oppenheimer was, in essence, blacklisted for spouting anti-American rhetoric and wasn't officially cleared of those charges until late last year.

"There’s themes in this film about being held up in this rarified air for something that you’re so conflicted about," Downey noted in a recent conversation with Extra TV (see below).

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Sitting down for the interview alongside co-star Cillian Murphy, the Marvel Studios veteran explained that while Nolan's World War II thriller initially "scared" him from an acting standpoint, he simply couldn't pass up the opportunity to play a supporting role for once.

"It was ... nice to have the pressure off because I let this guy [Murphy] do the heavy lifting," he said. "I’d come in and [go] ‘I had four days off. I was just antiquing around Santa Fe. What have you been up to?’ And he’d be like, ‘Uhhh…you wanna run the lines?’ He was also at all the locations where there were like 60-mile-an-hour winds and all that stuff. I hung out for the pleasant fair, weather-wise."

Watch Robert Downey Jr. & Cillian Murphy Discuss Oppenheimer

When asked about Strauss's dynamic with Oppenheimer, Downey recalled how Nolan cited the famous rivalry between two musical legends: Wolfgang Amadeus Mozart and Antonio Salieri. "It is a story of how small, imagined sleights between important people can have big impacts," the actor said, while also praising Murphy's take on Oppenheimer as "a generation-defining performance."

As for what audiences can expect from the Obadiah Stane-y Strauss, Downey stated that he was asked to tone down his usual snarky and motor-mouthed swagger approach that made him so lovable as the MCU's founding Avenger for 10 years.

RELATED: Oppenheimer Star Cillian Murphy Says Playing Father of Atomic Bomb Both 'Terrifying and Exciting'

"I’m always down for getting spartan and stripping things away," he conceded. "It’s not easy because I’m a little bit of a Mexican jumping bean by nature. But it was a gift and being able to do all these scenes with you [Murphy] as you carried this film as exceptionally as you did was just an honor."

Drawing upon American Prometheus (the massive, Pulitzer Prize-winning biography written by Kai Bird and the late Martin J. Sherwin), Oppenheimer arrives exclusively on the big screen Friday, July 21 in a number of different formats. Click here for tickets!

Want more from Robert Downey Jr.? Check out his conversational skills with the entire animal kingdom in Doolittle — now streaming on Peacock!