This is conduct unbecoming of Captain America.

Oh, stars and their goofy antics. Clowning on each other like a bunch of dopes… they're just like us, except infinitely more attractive. It's important for a major tentpole movie with a large cast like Avengers: Age of Ultron to establish that all the stars are good friends. Fans wouldn't be as inclined to see it if they thought they were just going to see superheroes pretending to like each other. And then the movie might only make a billion dollars.

Sources: Ellen