Of his aims for the first film, Whedon says "I want to know what makes them tick, what makes them flawed, what makes them fight - and ultimately, what makes them awesome. I go to these movies for those moments when the heroes define themselves, either through action or deliciously overwritten speeches."
How could a sequel possibly top that, by going bigger? "By not trying to. By being smaller. More personal, more painful... By being the next thing that should happen to these characters, and not just a rehash of what seemed to work the first time. By having a theme that is completely fresh and organic to itself."