Peggy Carter

Another Marvel role, another missed opportunity for Emily Blunt to join the MCU.

Producers had eyed The Devil Wears Prada standout for Peggy Carter, Captain America: The First Avenger's female lead, but she later explained to Vulture of Peggy and Black Widow not working out, "It was never the right time, really, and it just didn't work out scheduling-wise with those two. It's always a difficult thing to talk about, because it's not fair to the actresses who ended up playing them, you know? It just wasn't the right time."

While the role easily could've been a generic "girlfriend to the hero" one, relatively unknown Hayley Atwell, then just 22, turned Peggy into a fan-favorite.

"I likened her character to that famous Ginger Rogers quote. She can do everything Captain America can do, but backwards and in high heels," she said. "She's an English soldier through and through, although she always looks fabulous. She might stand there with a machine-gun shooting Nazis, but she's obviously gone to the loo beforehand and applied a bit of lipstick. She doesn't need to be rescued. That's exciting to me: her strength."

In fact, Atwell was so well-received she ended up getting her own TV show, Agent Carter, which ran for two seasons on ABC.



War Machine/Colonel Rhodes

There's no drama quite like MCU recast drama, as we learned with the Hulk. In Iron Man, Terrence Howard played Rhodes, U.S. Air Force officer and Tony Stark's BFF, who eventually becomes War Machine. But by 2010's Iron Man 2, Howard was on the outs with Marvel, leading to Don Cheadle taking over the role.

But Howard, who was the first actor cast in Iron Man (and was reportedly paid more than Downey) did not leave the franchise quietly; during an appearance on Watch What Happens Live, the vocal Empire star blamed RDJ for his exit.

"It turns out that the person that I helped become Iron Man, when it was time to re-up for the second one took the money that was supposed to go to me and pushed me out," Howard alleged. "We did a three-picture deal. So that means, you did the deal ahead of time. It was going to be a certain amount for the first one, a certain amount for the second, certain amount for the third." He then alleged they only offered him a small fraction of what he was originally intended to receive "'because we think the second one will be successful with or without you.' And I called my friend that I helped get the first job, and he didn't call me back for three months."

As for Cheadle, he didn't have a lot of time to make a decision about taking over War Machine's duties, revealing he was only given an hour to decide. "I said, 'An hour? It's like eight movies!'" he said on Late Night with Seth Meyers.

He also revealed that he nearly landed the role right from the beginning, telling MTV News, "I met very early on before the first one with Jon [Favreau] and the team...and I guess there was a split—some people Terrence, some people me."



Thanos

OK, technically, he's the biggest of the MCU's big bads thus far, but in his own mind, Thanos is humanity's savior, right?

While mostly CGI, directors Joe and Anthony Russo eventually sought out Josh Brolin for the role when it came time for Thanos to truly eff s--t up in The Avengers: Infinity War. (Thanos first made his debut in the post-credits scene of The Avengers, with Damion Poitier playing him.)

"He's such an amazing performer and so uniquely suited to Thanos," Anthony Russo said in an interview with The Telegraph. "What we were looking for in the character of Thanos is the strongest, most intimidating figure in the universe. While at the same time we wanted to tell a story about a character with a really complex and empathetic interior life. There are not a lot of actors that can give you both of those things. Josh Brolin is perhaps the best example that you can find where you have somebody who has a physical presence and brings that level of intensity and threat, while at the same time having another layer always at work underneath that, which is a very complex inner life and a lot of emotional depth and texture."

Joe added, "Josh created an incredibly nuanced character who is both frightening and oddly emotional."

When it came to making the decision to play the purple 8-foot-tall "Mad Titan," Brolin told Total Film he had to make a call first: "At the beginning, before I said yes, I called Mark Ruffalo and I said, 'What do you think of this?' He was like, 'It's going to be really uncomfortable in the beginning, and then you see the movie, and you'll just be absolutely blown away by the stuff you were doing.'"