Robert Downey Jr. is “so immersed” in creating Doctor Doom.

The 59-year-old actor is returning to the Marvel Cinematic Universe as the Fantastic Four villain and directors Joe and Anthony Russo told how the star, whose previous MCU character, Tony Stark/Iron Man was killed off in Avengers: Endgame, is intensely involved in working on his new alter ego.

Speaking to Entertainment Tonight, Joe Russo said: “It’s a very intense process developing the character. He’s so immersed in it. He is so dialed in.”

Anthony Russo added: “That’s the kind of artist he is. That’s the kind of actor he is.”

Joe continued: “[Downey’s] writing backstory, costume ideas ... I think he just loves really rich three-dimensional characters and I think he sees a real opportunity here with that character.”

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As well as Downey and the Russo brothers, it was previously reported Chris Evans, who first played Captain America in the MCU from 2011’s Captain America: The First Avenger until Endgame eight years later, could be returning for Doomsday, though it was not suggested which role the actor would be playing after he appeared as Fantastic Four star Johnny Storm, AKA the Human Torch, in Deadpool and Wolverine.

However, he later shot down the speculation and insisted he was “happily retired” from the superhero world.

Evans told Esquire: “That’s not true, though. This always happens. I mean, it happens every couple years —ever since Endgame. I’ve just stopped responding to it. Yeah, no — happily retired!”

However, fellow Marvel star Anthony Mackie is convinced fans will see Evans back in action again.

He told the publication: “I didn’t know! I talked to Chris a few weeks ago and it wasn’t on the table then. At least, he didn’t tell me it was on the table, because I asked him. I was like: ‘You know, they said they’re bringing everyone back for the movie. Are you coming back?’ He goes: ‘Oh, you know, I’m happily retired.’”

"I learned that right there. My manager showed me. He’s like: ‘Oh, so I guess Chris is coming back.’ That’s all I know. I haven’t seen a script.”

Avengers: Doomsday is scheduled to open in theaters on May 1, 2026.