Asked by ABC's Good Morning America if he was privy to the full top secret script, Mackie said, "I got 25 pages, which was the final fight sequence."
"And I sent a long email, like, 'this movie is going to be awful, how does this work, there's no story, this is gonna be the longest fight sequence ever,'" Mackie joked.
Generally speaking, the oft-given rule of thumb for screenplays is that one page equals one minute of screen time — suggesting a near-30 minute action-packed climax for what Mackie's co-star Chris Pratt called "the biggest movie of all time."
Earlier this year, Mackie teased a massive sequence featuring "like 40 superheroes," with Captain America actor Chris Evans calling the on-set moment "a great day with a lot of great people."
Directors Anthony and Joe Russo, who also helmed Captain America: Civil War, promise Infinity War boasts a showdown that dwarfs even that film's splash page-like airport battle.
"We have an equivalent that's like, if you had a comic book and you opened it up to your double panel, and then you fold it out, and then you folded it out again and again," said Joe Russo, miming a lengthy unfolding process.
Added Anthony Russo: "And then you got another comic book out and laid it next to it."
Marvel Studios chief Kevin Feige describes the Avengers threequel as the culmination of the shared Marvel Cinematic Universe after a decade of movies, all of which have been building to this.
"Avengers: Infinity War is by far the most ambitious movie we've ever made," Feige said. "It really is the start of the culmination of, at this point, 19 Marvel Cinematic Universe films and will feature almost every hero that we've introduced into the universe all in one movie."
Co-writer Stephen McFeely called the movie "ridiculously big," promising an "epic level of storytelling" as Earth's mightiest heroes assemble in an attempt to thwart the universe-shattering plans of alien warmonger Thanos (Josh Brolin).
ComicBook.com has teamed with Infiniti to give fans a chance to attend an early screening of Avengers: Infinity War ahead of its April 27 release.