Robert Downey Jr. always believed he was the perfect person to play the role of Tony Stark/Iron Man—the star of the first Marvel Cinematic Universe film—but as we learned later on, he was in the minority. Due to his longtime struggle with drugs and alcohol and brushes with the law, Downey was considered a risky pick for the role and the studio wasn't willing to roll the dice on him. Ultimately, it was Iron Man director Jon Favreau who was able to sway the studio to his way of thinking—which is fortunate, because it's impossible to imagine the MCU without Downey today.

It turns out that had another actor been cast, one of Iron Man’s most iconic lines would have been completely different, too, as it was improvised by Downey during production.

MCU fans will never forget the final scene of the first Iron Man movie, as it perfectly caps off the film and sets us up for Iron Man’s future. “I am Iron Man,” Tony Stark states at a press conference, admitting his superhero identity to the world.

The line became so iconic that it appeared again at the end of Iron Man 3. And Downey’s improvisation actually turned out to have a wider impact on Marvel Studios as a whole.

"That success inspired us to go further in trusting ourselves to find the balance of staying true to the comics and the spirit of the comics but not being afraid to adapt and evolve and to change things," Marvel Studios president Kevin Feige told Deadline in 2018.

In the comic books, it takes several years for Stark to confirm his alter ego to the public.

“It’s a fine line,” Feige continued. “Tony Stark not reading off the card and not sticking with the fixed story? Him just blurting out ‘I am Iron Man?’ That seems very much in keeping with who that character is. It just hadn’t been done in the comics before, but it was something very much in keeping with the comics’ character and what he could have done.”

More than 10 years after the release of the original Iron Man, Feige remains the architect of the MCU, and therefore one of its key decision-makers.

“I think it did inspire us on all the movies," Feige continued. "What I love now—20 movies in—is how fans expect the MCU to change and adapt. They expect us to be inspired by the comics as opposed to being slavishly devoted to them."

Fans will see Downey don his Iron Man suit once more when Avengers: Endgame hits theaters on April 26, 2019.