Throughout his near-decade of service in the Marvel Cinematic Universe, with seven starring roles and a few cameos over eight years' time, Chris Evans' Captain America was always far more likely to deliver a rousing inspirational speech than a wisecrack. Beloved by his fictional peers and real-world audiences alike for his sincerity and loyalty, being a square-jawed square with an unwavering moral compass also made him the butt of jokes — which is why the recurring bit about his owning "America's ass" in this spring's Avengers: Endgame became such a breakout, memed moment.

Like many jokes, this one is told in three parts, though that wasn't always the intention. At first, it was just a one-off quip from Tony Stark, one of the endless cracks he makes even in the most extreme circumstances.

"It sort of grew on its own," Endgame co-writer Christopher Markus told SYFY WIRE this week. "The first thing that was there was Tony looking at Cap's terrible Avengers outfit and going, 'that suit does nothing for your ass.' That stood on its own for a while, and then Ant-Man was added to the scene, and he said, 'I think that's America's ass.' And that was the joke for a while."

It was only much later, during film reshoots, that it was decided that Cap himself would get in on the gag.

"We redid the Cap on Cap fight, the ending of it, and that helped us tremendously," co-writer Stephen McFeely explained, referring to a showdown between 2012-era Captain America and his time-traveling self, a fight waged over the Mind Stone. "And then we needed to get out of that scene a little differently, and there [2012 Cap] was, slumped on the ground with his butt in the air."

As Markus explains it, the writers asked themselves a crucial question: "Can we put another ass in the movie?"

The answer was clearly yes. So as he steps over his knocked-out past self, the modern-day Captain America stares down at his behind and delivers one of the biggest laugh lines of the film: "That really is America's ass." And he says it with a smile, a reflection of Evans' own feeling on the line.

Video of "That's America's ass" - Avengers: Endgame

"He was certainly okay with calling attention to that outfit," McFeely said, laughing. "It wasn't his favorite."

"It's rare that you can find a joke for Captain America to tell that [is] in-character as Captain America because he doesn't do a lot of non-sequitur smart remarks," Markus added. "So I think Chris is happy when you can get something funny that stays in the Cap wheelhouse."

That the actor was very willing to make the crack is notable because he's long been so protective of the character. It wasn't that Evans didn't want his Captain America to have flaws or make bad choices, but he did feel that every single line spoken had to be in line with his gradual development and steadfast principles.

McFeely and Markus wrote every single movie with "Captain America" in the title, as well as the last two Avengers films, so they worked with Evans more than anyone. Together, they plotted out his rise from skinny kid in Brooklyn to World War II super-soldier and Avengers leader, to rebel and defector, and then back to leader of the super-team in his final outing. He was the beating heart of the franchise and the centerpiece of Endgame, which made nearly $2.8 billion worldwide this spring and summer.

"Chris knows Cap really well," McFeely said. "We all have come to sort of weigh in on him, but he more often cuts things than adds things for sure. Just like, 'I don't need these three sentences. I just need these two.'"

For the record, "That really is America's ass" is just one sentence, and it was very effective.