Robert Downey Jr. cried when the Russo brothers told him of Tony Stark's fate in 'Avengers: Endgame'

• Robert Downey Jr. cried when he learned what Tony Stark's fate would be in "Avengers: Endgame."
• Directors Joe and Anthony Russo spoke about the moment in a new Marvel behind-scenes book.

Robert Downey Jr. cried when "Avengers: Endgame" directors Joe and Anthony Russo told him their plan for Tony Stark/Iron Man's fate in the 2019 blockbuster movie.

"When we started winding down the pitch, and getting to his death, Robert started crying," Joe said in a new behind-the-scenes book called "The Story of Marvel Studios: The Making of the Marvel Cinematic Universe" by Tara Bennett and Paul Terry.

He continued: "And when we were done, he said, 'That's fucking awesome.' That's when we knew that we had to do it. Because he felt it."

In the book, released on October 19, the Russos recalled meeting with Downey Jr. shortly after the release of "Captain America: Civil War" in 2016 to discuss the direction they had in mind for the character.

"In a way, it's almost like a death in the family," Joe said of meeting with the actor specifically to tell him about their idea for Tony to sacrifice himself. "When you're saying to someone, 'All these people you've loved and have hung out with for 10 years? That's all gonna go away because we want to tell a story a certain way.'"

"What we wanted to do is communicate the entire pitch to Robert, to sort of take his temperature on it, because he had so much creative ownership over the character," Anthony said. "We were like, 'If Robert gets this, if Robert feels this, then we know we're in a good place.'"

With Downey Jr.'s approval, Tony made the ultimate sacrifice near the conclusion of "Endgame" as he wielded the power of the six Infinity Stones to dust the villain Thanos (Josh Brolin) and his army. The victory was bittersweet because, in snapping his fingers, the radiation and energy killed Tony.

Downey kicked off the MCU with his portrayal of the billionaire-turned-superhero in 2008's "Iron Man."

The standalone film spawned over a decade of interconnected films, building up to Downey Jr.'s swan song in the 2019 movie "Endgame."

Over the years, Downey Jr. has integrated his own creative ideas into the various Marvel projects he's participated in.

The actor provided insights that were beneficial to the Russo brothers as they worked to fine-tune "Civil War," the directors revealed in "The Story of Marvel Studios."

Then in Tom Holland's screen test for the role of Spider-Man, he went off-script with the young actor. The two later improvised an awkward hug misunderstanding in a car in "Spider-Man: Homecoming" as well as Peter Parker's heartbreaking dusting in "Avengers: Infinity War."

Downey Jr. even inspired his character's "I love you 3000" line spoken to his daughter, Morgan Stark (Lexi Rabe), in "Endgame."