Although she’ll be working with Iron Man to a degree, Johansson informed EW that this is more so that she’s in a position where she can let the two sides duke it out and get a sense of what’s really happening. The actress said:
I think when you find her in Civil War, she’s looking to strategize her position, putting herself in a place where she is able to let the powers that be fight it out or whatever amongst themselves. She’s always a little bit on the perimeter so she can have a better perspective of what’s really going on.
Although the Sokovia Accords are targeting all of the Avengers, Black Widow is among the few that don't have superpowers and isn't armed with crazy tech, so that gives her a unique perspective on Earth’s Mightiest Heroes being policed by the world’s governments. By publicly siding with Tony Stark, that makes it so she won’t be arrested or be forced to go on the run, but she’ll also have enough distance to look at what’s unfolding and see what can be done.
While Black Widow won’t escape physical confrontations altogether in Captain America: Civil War (the D23 footage showed her tangling with Hawkeye), Johansson also mentioned that because she’s not truly allied with one side, that will let her fill a “mediator” role within the story. She added:
You know, I think she understands where everyone is coming from. And none of it really matters to her, you know? There’s a bigger problem at hand and she’s, I think, strangely, kind of the mediator. Which is not exactly how you would imagine her to be. But I think she really does see both sides of the coin and I think her strength is that she’s not personally involved.
Being a spy for most of her life, Natasha’s main skill set has been obtaining information and taking out any obstacles in her way, so for her mediating is a new line of work. It also helps that she’s close with both of the factions’ “leaders.” She and Tony built a relationship during Iron Man 2 and as Avengers teammates, but she has a rapport with Steve from when they worker together at S.H.I.E.L.D. Of course, if she finds herself in a position to get these two to try and make up, she’ll have her work cut out for them, as eventually those two will be throwing down.
We’ll see if Black Widow has something up her sleeve when Captain America: Civil War hits theaters on May 6, 2016.