Robert Downey Jr. is returning to the MCU not as Tony Stark, but as the fearsome Doctor Doom in the upcoming Avengers: Doomsday.

However, it sounds like the resemblance between Doom and Tony won't be a coincidence, but will actually factor into the movie.

"No better actor in the world than Robert Downey," Joe Russo told Soundsphere, when asked about Downey Jr., and Tony and Doom looking the same. "I'm not going to give away anything. It will all make sense when you watch the movies.

"What we love about telling these stories is that they're serialised. This is a giant experiment," he continued. "Nobody's ever done anything like this before. Marvel's unique. You're taking all these different franchises and combining them into overarching stories. If you're a fan of the comics like I was, I still have my comic book collection. I'm 54 years old and the idea of… That's shifting and changing and surprising you and then reinventing itself and then shifting and changing and then surprising you. That's exciting.

"So get ready for it. We were with Robert earlier and we're both talking about this concept that we are back to phase zero. This is starting over from scratch. We want to make sure everybody feels like we're not leaning on anything from the past. This is going to be a you’ll see when you see the movie, a complete new direction," he concluded.

Added Anthony Russo: "You know what an incredible performer Robert is, and it's a brand new character that you can't even guess at."

Along with Downey Jr., the supersized cast includes the likes of Chris Evans's Steve Rogers, Chris Hemsworth's Thor, Anthony Mackie's Captain America, the Thunderbolts, the Fantastic Four, and the Fox-era X-Men.

Avengers: Doomsday arrives this December 18. While you wait, check out our guide to all the upcoming Marvel movies and shows for everything else the MCU has in store.