Captain America: Civil War, starring Robert Downey Jr as Tony Stark and Chris Evans as Steve Rogers, will see the two superheroes take on each other.
In an interview with the Empire, Downey Jr has revealed the real reason behind his decision on signing the film.
He said, "They (Marvel) said to me, 'If we have you, we can do this, or 'Cap 3' has to be something else.' It's nice to feel needed. And at this point it's about helping each other, too. I look at it as a competition and I go, 'Wow, maybe if these two franchises teamed up and I can take even a lesser position, with people I like and directors I respect, maybe we can keep things bumping along."
The Iron Man also hinted at the origins of discord between Iron Man and Captain America, which will be further explored in Civil War.
He said, "It's natural to change your views. The main thing to me is, what sort of incident could occur, and what sort of framework could we find Tony in? The clues about where we might find him next are in Ultron."
"But what would it take for Tony to completely turn around everything he's stood for? Joss brings this up all the time. It's kind of weird that these guys would have all these throw downs all over planet Earth and yet when the movie's over, nobody minds. What would the American government do if this were real? Wouldn't it be interesting to see Tony doing something you wouldn't imagine?" he added.
However, the actor doesn't necessarily believe that Tony Stark's character transformation will be villainous like in the comics.
"I wouldn't put it that way. The biggest question is, for Chris and for Cap, how do we bring Cap to a place where people go, 'Man, I never thought I would see such a vast change in Steve'. After you see Snowpiercer, you're like, 'I want to see a little bit of that guy'," he explained.
Captain America 3 will follow the adaptation of the famous Marvel Comics Civil War storyline that saw Steve Rogers and Tony Stark take on each other over the government's plan to force superheroes to registering their real identities.
Captain America: Civil War releases in theaters on 6 May, 2016.