Iron Man is the first film in the Marvel Cinematic Universe, and a decade after its release we still look to it for a lot precedents other MCU films have stuck to: The blend of comedy and action, the post-credits sequences, the frequently dead villains, and so on. Iron Man also helped set the tone for the MCU in a more subtle way, though, and it came at the very end of the film, when Robert Downey Jr. decided to go off script and just blurt out "I am Iron Man."
In the film's final scene, after the villainous Obadiah Stane (Jeff Bridges) has been defeated, Tony Stark is all set to speak at a press conference concerning what just went down at Stark Industries. Before he takes the stage, he's handed an alibi by S.H.I.E.L.D. agent Phil Coulson (Clark Gregg): He was on his yacht when the incident happened, fake travel papers and all, and the man in the "iron" suit (as Stark points out, it's really a gold-titanium alloy) was his bodyguard. This is not only a way of preserving his secret identity for the films, but it's also true to the comics, as Iron Man was publicly viewed as "Tony Stark's bodyguard" for years in the Marvel Universe.
When it came time to shoot the moment, though, Downey added one of his many improvisations to the film. He looked at the cards, hesitated, and said "I am Iron Man." Director Jon Favreau and producer Kevin Feige could have insisted on using another take, with the alibi intact, in the finished film. In a new interview with Deadline to celebrate the film's 10th anniversary, Feige both revealed the ad lib and explained why it was important to take a chance, and how that chance served the MCU going forward.
“That success inspired us to go further in the trusting ourselves to find 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,” Feige said. “It’s a fine line. If you’re changing something for no reason, that’s one thing, but if you’re changing something because you want to double-down on the spirit of who the character is? That’s a change we’ll make. 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. I think it did inspire us on all the movies. 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.”
That approach -- inspired by as opposed to word-for-word based on the comics -- has served the MCU well, from disposing of the "Dr. Donald Blake" alter ego in Thor to avoiding Spider-Man's origin story to tweaking the motivations behind the inter-superhero battle in Captain America: Civil War. It allows the films to be both recognizable and fresh simultaneously, and while we can't hinge every single one of those choices on Downey's moment of inspiration, it did help lay the groundwork for a universe still capable of surprising us today.