Joss Whedon Talks Casting James Spader in The Avengers: Age of Ultron
"Spader was my first and only choice. He's got that hypnotic voice that can be eerily calm and compelling, but he's also very human and humorous. Ultron is not Hal. Spader can play all of the levels. He's the guy to break The Avengers into pieces."
"Obviously, Age of Ultron suggests that in The Avengers 2 there will definitely be some level of, if not giant evil robots, then definitely a robotic or artificial intelligence entity. I may or may not have been excited about it for a year. Not every aspect of Phase Two is planned already, but Joss has been thinking about it for a year and Kevin Feige has been thinking about what The Avengers 2 does since the first film. The scale I think they're going to reach for is going to be truly epic, which is incredible when you consider how epic Marvel's The Avengers was. But on a character level the story will actually be smaller, or at least more intimate. It's hard to put a list together of all the things Joss is good at as a writer, but definitely in the top five would be dealing with the shifting dynamics of a large group of characters."