With Phase Two of the Marvel Cinematic Universe in full swing, it’s almost time for Joss Whedon to assemble Earth’s Mightiest Heroes for principal photography on The Avengers: Age of Ultron, and speaking to SFX Magazine (via CBM), Marvel Studios chief Kevin Feige has offered up some details on the eagerly-anticipated sequel.

“Joss Whedon had a great idea for the character and how to use him to unify the Avengers again and acquire their services once more,” said Feige with regards to the choice of Ultron (James Spader) as the main villain for the sequel. “He was a favorite. Joss and I were on the production of the first Avengers movie and we started having conversations about Ultron, and how he could come about and how we could adapt his origin to the continuity of the Cinematic Universe. It literally was just little discussions between takes but thankfully Joss files all that kind of stuff away in his brain. Then when the time arrived, he started outing it, so he went back to that and built a very good story… I first met James Spader two or three years ago. I thought it would be great to have him in our universe and we finally found the right role with Joss’s version of Ultron.”

James Spader isn’t the only addition to the cast, and Feige also went on to talk about the introduction of Quicksilver and Scarlet Witch, played by the Godzilla duo of Aaron Taylor-Johnson and Elizabeth Olsen: “It’s always a balance with any movie, not to overwhelm it with too many story elements. Certainly when it comes to comic book movies, you can fall into the trap of having too many villains and in the case of team-based movies, having too many heroes. But what Joss has in mind to bring those characters into the story happens in a very natural fashion over the course of the story. It’s also the tradition of the Avengers to switch up the roster every now and again and to have new characters coming in. It’s just part of the fun, but we wouldn’t have included them if it had just been a case of ‘Oh, we want two new figures, two new costumes.’ It very much flows with the story that Joss is creating.”

The Avengers: Age of Ultron is set for release on May 1st 2015 with a cast that includes Robert Downey Jr. (Iron Man), Chris Hemsworth (Thor), Chris Evans (Captain America), Scarlett Johansson (Black Widow), Jeremy Renner (Hawkeye), Samuel L. Jackson (Nick Fury) and Cobie Smulders (Maria Hill).