This story contains certain key spoilers regarding the events of "Avengers: Endgame"

(CNN) One would imagine that as the screenwriting duo of Christopher Markus and Stephen McFeely began to tackle their sixth and, for now at least, final film set in the Marvel Cinematic Universe, "Avengers: Endgame," they would've walked into the assignment with all the swagger of Tony Stark, or the resolute righteousness of Steve Rogers.

But one would be wrong. Even after the wild box office successes and critical praise of their previous efforts in the MCU -- including all three "Captain America" films, "Thor: The Dark World" and "Avengers: Infinity War," the penultimate episode in the ten-year interconnected superhero saga -- the pair was very aware that the highly anticipated concluding chapter was as much theirs to screw up as to win the day.

"You're on a high wire falling on your face, where it's going to either get bogged down in such a sense of your own importance that you forget to make an entertaining movie, or you're so busy dotting the I's that you forget to grip people emotionally during the run of the movie," Markus told CNN of their initial "Endgame" jitters. "It was a constant sort of calibrating and rejecting and going over it and making sure that we weren't just taking a victory lap. Because that is death."

Their commitment, of course, would pay off in spades: "Endgame," which has already made its debut on digital download and bows on physical home media on Tuesday, was instantly embraced by both fans and critics, becoming the top-grossing film of all time.

'Avengers: Endgame'

"I guess maybe Marvel makes it look easy, because they are fun [movies], but when Chris and I took the job, we did it very carefully knowing that it was going to take a lot out of us and it was going to be the biggest puzzle that we ever faced," said McFeely. "The amount of plates we're juggling, the amount of storylines, being able to bring them to the conclusion, and yes, making sure that we're honoring 20-ish other movies in the process, was rather complicated."

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