Many fans were surprised to find that Ultron's creator in Avengers: Age of Ultron would be Tony Stark, rather than Hank Pym, who created the villain the comics. In a previously unpublished portion of an interview with Empire Magazine in April 2014, director Joss Whedon explains why that decision was made.
“Of all the heat I’ve ever taken, not having Hank Pym was one of the bigger things,” says Whedon. "But the fact of the matter was, Edgar [Wright] had him first and by virtue of what Edgar was doing, there was no way for me to use him in this. I also thought it was a bridge too far. Ultron needs to be the brainchild of the Avengers, and in the world of the Avengers and the MCU, Tony Stark is that guy. Banner has elements of that guy – we don’t really think of him as being as irresponsible as Tony Stark, but the motherf**ker tested gamma radiation on himself, with really terrible, way-worse-than-Tony-Stark results."
“It didn’t make sense to introduce a third scientist, a third sciencetician, to do that,” Whedon continues. “It was hard for me, because I grew up on the comics, to dump that, but at the end of the day, it’s a more interesting relationship between Tony and Ultron if Tony was once like, ‘You know what would be a really great idea?’ They’re doing what they always do – which is jump in headfirst, and then go, ‘Sorry, world!’ But you have to make it their responsibility without just making it their fault.”
Whedon also talked a bit about toning Ultron down a bit for the film, in order to make him more of a character.
“The powers in comic books – they’re always like, ‘And then I can reverse the polarity of your ions!’ – well, we have to ground things a lot more,” Whedon explains. “With Ultron, we have to make him slightly less omnipotent because he’d win. Bottom line. Also, having weaknesses and needs and foibles and alliances and actually caring what people think of him, all these things, are what make him a character and not just a tidal wave. A movie about a tidal wave can be great, but it’s different than a conflict between one side and the other.
“When Ultron speaks, he has a point. He is really not on top of the fact that the point he’s making has nothing to do with the fact that he’s banoonoos. And that he hates the Avengers for bringing him into this world, and he can’t really articulate that or even understand how much he hates humanity. He thinks he all that. That guy is very fun to write. He combines all the iconic stuff. The powers he has are slightly different – he can control certain things, he’s not just firing repulsors.”
Avengers: Age of Ultron opens May 1.