Collider is running a multi-part interview with Avengers: Endgame editor Jeff Ford, who Joe and Anthony Russo credit with giving Tony Stark the perfect 11-year cap to his run as Tony Stark by having him go out, looking at Thanos, Infinity Gauntlet on his hand, and saying, "I am Iron Man."
For a while, though, it wasn't going to be that way at all.
Jeff mentions that Robert Downey Jr.'s acting process is one of discovery, which gives the editor a plethora of choices when cutting the movie together.
When we were putting together the end of the movie, when we shot Tony's last moment in the first round, we shot a bunch of different options. Robert had different ideas…We give him space to do that. Joe and Anthony are great about improv. We shot a run of different performances for that last moment. Some of them were crazy. Some of them we would never have used.
For a long time, though, there was an edit that the creative team preferred and this frontrunner was not what ended up on the screen. In this alternate version, the final exchange between Thanos and Stark was pure visual storytelling without dialogue.
"In that version, Thanos didn't say anything either. He had the gauntlet. He looked at him like, 'I got you.' Snapped. Looked. Couldn't believe it. Turned to Tony. Tony raised his hand and snapped. It was beautiful. It worked really, really well. But what we found, though, was Thanos needed an arc in Endgame. That arc was his sense of inevitability. The story we'd been telling was that Thanos' pitch in that movie is 'no matter how many times you try and stop me, you can travel in time, you can do all these things, you're never going to win.' It's a sense of destiny, of 'I will always be the one who wins.' They're trying to undo destiny. They're going against what happened. For the movie to have thematic coherence, the end of the movie needed to be Thanos saying, 'I told you. You cannot win,' and for Tony to say, 'But we can'.
And thus was born the version where Stark beats Thanos with a snap and the words that, at the end of his first film, launched the MCU.