"Because there was a book called Age of Ultron quite recently, a lot of people have assumed that is what we're doing, but that is not the case," states Whedon (via ComicBookMovie). "We're doing our own version of the origin story for Ultron. In the origin story, there was Hank Pym, so a lot of people assumed that he will be in the mix. He's not. We're basically taking the things from the comics for the movies that we need and can use. A lot of stuff has to fall by the wayside."
Whedon then went on to explain a little bit more about his plans for Ultron's origin, seemingly indicating that Tony Stark could be responsible for his creation: "We're crafting our own version of it where his origin comes more directly from The Avengers we already know about. It's a little bit darker than the other film because Ultron is in the house. There's a science fiction theme that wasn't there in the other one. Ultron is definitely something that evolves, so we're going to get together a couple of different iterations. Nothing can be translated exactly as it was from the comics, particularly Ultron."
And finally, he offered a few more details about the film, suggesting we can expect to see an increased role for Jeremy Renner's Hawkeye in the sequel: "It's very much a global Avengers film. A lot of the movie has to do with their place not just in America, but the world. Part of the fun for me, definitely this time around, is writing Hawkeye. He did get possessed pretty early by a bad guy and had to walk around all scowly for most of the movie so now it's nice to actually have the character there and see him interact with the other guys."
Avengers: Age of Ultron is set for release on May 1st, 2015, with Robert Downey Jr. (Tony Stark / Iron Man) confirmed as returning and Chris Evans (Steve Rogers / Captain America), Chris Hemsworth (Thor), Scarlett Johansson (Natasha Romanoff / Black Widow), Mark Ruffalo (Bruce Banner / The Incredible Hulk), Jeremy Renner (Clint Barton / Hawkeye), Cobie Smulders (Maria Hill) and Samuel L. Jackson (Nick Fury) all expected to reprise their roles. Meanwhile, Earth's Mightiest Heroes will also have (at least) two new members in the siblings Scarlet Witch and Quicksilver, with Kick-Ass and Godzilla star Aaron Taylor-Johnson strongly tipped for the role of the silver-haired speedster.