A self-described genius, billionaire, playboy and philanthropist with a mechanical suit of armor of his own invention. Downey was cast as part of his four-picture deal with Marvel Studios, which includes Iron Man 2 and The Avengers. Downey stated that he initially pushed Whedon to make Stark the lead revealing, "Well, I said, ‘I need to be in the opening sequence. I don't know what you're thinking, but Tony needs to drive this thing.' He was like, ‘Okay, let's try that.'
We tried it and it didn't work, because this is a different sort of thing, the story and the idea and the theme is the theme, and everybody is just an arm of the octopus." About the character's evolution from previous films, Downey commented, "In Iron Man, which was an origin story, he was his own epiphany and redemption of sorts. Iron Man 2 is all about not being an island, dealing with legacy issues and making space for others... In The Avengers, he's throwing it down with the others".
A World War II veteran who was enhanced to the peak of human physicality by an experimental serum. Evans was cast as part of a deal to star in three Marvel films, in addition to The Avengers. Evans stated that Steve Rogers is much darker in The Avengers, explaining, "It's just about him trying to come to terms with the modern world. You've got to imagine, it's enough of a shock to accept the fact that you're in a completely different time, but everybody you know is dead. Everybody you cared about... He was a soldier, obviously, everybody he went to battle with, all of his brothers in arms, they're all dead. He's just lonely. I think in the beginning it's a fish out of water scene, and it's tough. It's a tough pill for him to swallow.
Then comes trying to find a balance with the modern world." Regarding the dynamic between Captain America and Tony Stark, Evans remarked, "I think there's certainly a dichotomy-this kind of friction between myself and Tony Stark, they're polar opposites. One guy is flash and spotlight and smooth, and the other guy is selfless and in the shadows and kind of quiet and they have to get along. They explore that, and it's pretty fun".