Speaking with Marvel LIVE at San Diego Comic Con after he announced that "The Avengers" sequel would be titled "The Avengers: Age of Ultron," writer/director Joss Whedon elaborated a bit more on why that title was chosen. He also spoke about the backstory of the character and how it is being changed for the film, including the removal of Ant-Man's involvement:
"A lot of people, because we called it Age of Ultron and there was a book called Age of Ultron recently, assumed that was the storyline we were doing which is not the case. We're doing our own version of the origin story of Ultron.
The other thing is in the origin story, there was Hank Pym, so a lot of people assumed he was going to be in the mix, but he's not because 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 way side. Ultron was their chief villain when I read the book. He has been THE Avengers villain for so long, I was amazed anybody was surprised it was going to be him in a movie. But we're sort of crafting our own version of it, where his origin comes more from The Avengers that we already know about it.
Since before I even took the job of making the first one, I thought 'Well I don't know if I want to make this movie, but the second one should have Ultron in it!'
Ultron is definitely somebody who evolves so we're going to get a couple of iterations, it's going to be cool."
Talking about the differences with the first movie, Whedon says the film has a bit more sci-fi in it, and one character finally gets to be himself.:
"It's a little bit darker than the other film, because Ultron is in the house there's a science fiction element that I think is a little bit stronger than the other film. Getting the team together was so rousing, but seriously, keeping the team together is a completely different problem. Part of the fun for me, definitely, this time around is writing Hawkeye, because he did get possessed pretty early by a bad guy and sort of had to walk around scowley for most of the movie last time. So now it's nice to actually have the character there and really see him interact with the other guys."
Finally, Whedon confirms that filming on the sequel begins late February/early March in the United Kingdom.