One of the most memorable scenes in Avengers: Endgame was seeing Tony Stark snapping his fingers and turning Thanos to dust with the last words "I am Iron Man". But Robert Downey Jr. was reluctabt to say the final line at first.
The ending of Avengers: Endgame was very emotional but not really shocking. We knew that either Iron Man or Captain America was going to die and it just happened to be Iron Man. But Tony's death was not in vain as he tool the Mad Titan along with him. And, the last words of the very witty and eccentric Tony Stark before snapping his fingers were "I am Iron Man!"
It was actually a great final line but Robert Downey Jr. was first very reluctant to film the scene. The scene which was added in during reshoots was pitched to Robert Downey Jr. by the Russos over dinner. However, RDJ didn't want to go back to film the scene as it was so emotionally draining and he did not want to go back to that place. Producer Joel Silver, thankfully, was able to save the day and convince RDJ to do the reshoots:
It’s an interesting story. I had dinner with [Robert Downey Jr.] like two weeks before we were supposed to shoot it. And he was like, ‘I don’t know. I don’t really want to go back and get into that emotional state. It’ll take… it’s hard.’ And crazily enough, Joel Silver, the producer, was at the dinner. He’s an old buddy of Robert’s. And Joel jumps in and he’s like, ‘Robert, what are you talking about? That’s the greatest line I’ve ever heard! You gotta say this line! You have to do this!’ So thank God that Joel Silver was at dinner, because he helped us talk Robert into doing that line.
In another interview, the Russo brothers explained that the "I am Iron Man" line was not originally part of the plan with Tony just supposed to snap his fingers without saying it. But the directors felt as though it was out of character for him to be silent.
Anthony Russo: Originally when he snapped, he didn’t say, ‘I am Iron Man.’ The idea came up while we were in post production. Our editor, Jeff Ford…
Joe Russo: We were sitting on our editorial, and we were reworking that sequence, and Thanos says ‘I am inevitable.’ And we were like, ‘We need a response to that. What is the response to that?’ And our editor said, ‘What about I am Iron Man?’ And we were like, ‘That’s it!’
Anthony: We must shoot that, we have to shoot that!
This is how it went down and we completely understand why RDJ was reluctant at first. He has been a part of the MCU for over a decade and it's crazy to think how he must have felt in that moment when his character died. But he went back in and did the reshoots for us and we love him even more now.