Robert Downey Jr. changed the Marvel Cinematic Universe forever by improvising a line in Downey's time in the MCU to a close.
Although it wasn't planned, Downey's final MCU film ended in a similar fashion as his first. The very end of Iron Man saw Stark it he was now a superhero by saying "I am Iron Man" to the press. This phrase was repeated at the end of Avengers: Endgame line was added at the last second, the original line at the end of Iron Man was improvised by Downey on set. With just this single line, he managed to change the MCU's future just as it was beginning.
The original ending for Iron Man isn't known, but revealing his superhero lifestyle to the public was not it. But when Downey improvised the "I am Iron Man" line, Marvel Studios kept it for the film. In doing so, the MCU began by ditching one of the biggest comic book tropes. The universe was launching at a time when Christopher Nolan's Dark Knight trilogy was as popular as ever, the original Spider-Man franchise had just ended, and Fox was still figuring out where to take X-Men next. Secret identities play a big part in most of these superhero franchises, which is why it became all the more surprising for Iron Man to end with Tony Stark's public reveal.
From there, the MCU grew into a superhero franchise that almost never deals with heroes with secret identities. When Chris Hemsworth's Thor was introduced a few years later, the MCU byed the comics' decision for him to pose as a regular human named Donald Blake when on Earth. Meanwhile, the identities of all the original Avengers are known to the world, with even T'Challa becoming known as Black Panther in his first appearance. Even with Spider-Man, a character whose secret identity has been a major part of his characterization in the comics, Spider-Man: Far From Home ended by outing his true identity. All of this is ultimately the result of Iron Man's ending. As a result, the MCU did in a single film what took decades to accomplish in the comics in some cases, only the films never had to retcon these decisions to make the identities secret again.
But, Downey's improvised line had an even larger impact on Marvel Studios beyond secret identities. Marvel Studios president Kevin Feige explained in the past that it emboldened his team with the confidence to not stay 100 percent faithful to the comics. The approach to adapt aspects from the comics but change and improve other pieces has been a big part of the MCU's continued success. Although there are certainly some fans who would prefer to see characters and events stay more faithful to the source material, Marvel has mostly done a terrific job of picking which parts of a character or story need to evolve. And to think, it's all because of an improvised line at the end of the first MCU movie.
