As for the lack of screentime devoted to Cap and Black Widow, Markus says it mostly came down to character:
Some of the people, when you weigh up who you are seeing, seem to take a little bit of a back seat in this one. When we did crack them, what we realized was they had far more potential in the movie coming next year, just in terms of how their character would be tested by the story. Im not telling you what the story is. I will say there is less Steve Rogers and Natasha (Scarlett Johansson) than you might expect in this movie [Infinity War]. Because they are such hard-bitten people, when a threat comes, they stand there and they take the threat. They dont crack. They dont whimper. They dont start talking about all their failed opportunities. So, there isnt that much to explore, especially when you dont have a lot of time. But theres a lot later.