Scarlett Johansson has already confirmed (just in case those set pics didn't already) that her character Black Widow and Captain America: TWS's titular Avenger share "something of an intimate friendship" in Marvel's upcoming sequel, but was that the studio's plan all along? If you go back and watch The Avengers, the two do actually shoot a couple of admiring glances at one another! Coming Soon put the question to the actress, who admits that "the pairing is strange on paper", and although "Marvel's really good at noticing those connections", it actually wasn't something that was always in the cards.
No, actually. Of course in "Avengers" particularly you start to see the relationships forming between these characters, and all of us have very specific relationships with one another. Of course we all know how we feel about one another. We work and all use each other in different ways. We all have very specific opinions about one another. I think that Joss laid the groundwork for a lot of interesting connections. That's why the audiences are so intrigued by that film because they love to see how these characters react to each other that come from completely different universes.
Read more at http://www.comicbookmovie.com/fansites/notyetamovie/news/?a=86952#Lds07Q1UDv2cHFqY.99