Captain America and Black Widow have been through a lot together in the Marvel Cinematic Universe. We've watched their relationship grow through several Marvel movies and now, they are two of the Avengers left standing following "the snap." Recently, directors Joe and Anthony Russo spoke about how Black Widow has evolved as a character over the course of the series, and Joe Russo revealed that the events of Avengers: Infinity War, combined with what she and Captain America have already been through together, is going to result in a pair of somewhat different characters than we're used to seeing. According to Joe Russo...
We've been fortunate enough to grow [Black Widow] over three films: Winter Soldier, Civil War and Infinity War. And in Winter Soldier, what we found so compelling about pairing her up with Captain America was that he is a service paragon of morality and she is a paragon of gray. But they've grown together, and I think when you find them in this movie and they've been the Secret Avengers for two years because they're being hunted by Ross and the government, they're like a tight-knit family now. Her, Sam Wilson, and Steve Rogers. It's hardened all of them in a way. You can't go on the run without it affecting you emotionally, so I think all of those characters, but specifically the ones who survived Infinity War, Cap and Natasha, you will see them with a much harder edge.
It's an interesting transition because, over the course of the MCU, we've watched Black Widow, who Joe Russo describes as a "paragon of gray" (can you be a paragon of a non-extreme?) be influenced by the other heroes around her, most notably Captain America, and go from being that morally ambiguous character to one that is more in line with the traditional view of the superhero. Now, the Russos' comments to Wired makes it sound like Captain America might be shifting more toward Black Widow's previous perspective. After being on the run together and now watching a bunch of their friends vanish, it sounds like Captain America may be feeling a little less like a paragon of morality. Steve Rogers has always been, not just the moral center of the MCU, but its heart as well. What does it mean if that heart has become cold?
One certainly expects the loss of half the life in the universe to have a profound impact on those that are left, but there's so much we don't know about how that will work in Avengers 4. Certainly, if Captain America and Black Widow have a harder edge to them they will be a formidable, and potentially dangerous foe to anybody standing in their way. Thanos better look out.