Via MTV, Whedon had this to say:
“Natasha is a huge part of the sequel because you do want to concentrate on the people who don’t have their own franchises. Although she in ‘Cap 2,’ [and] she’s great. She was the most fun for me because she’s not a hero, you know, and it’s something that I read—and I feel bad that I can’t remember who wrote the book—but it’s in one of the books explaining, ‘These guys are heroes, you are a spy. It’s a different thing—it’s a different skill set—and you don’t have their moral high ground or any of that good stuff.’ And that just makes her so interesting to me. So yeah, the stuff I’ve got going on with her in the second one is killer.”
Avengers: Age of Ultron will open in theaters on May 1, 2015.