One of the things that fans are going to be most excited by when they see "Avengers: Age Of Ultron" is the developing relationship between Bruce Banner and Natasha Romanoff. We see our first indications of it when she finds Hulk on the battlefield at the end of the opening action sequence, and she initiates "the lullaby," a technique they've developed for calming the Hulk and allowing Banner to regain control.
Overall, Mark Ruffalo and Scarlett Johansson do some of my favorite work in the new film, and not just in that great scene between the two of them in the middle of the movie. Ruffalo is playing both Hulk and Banner now, and there is some great stuff he does in both roles. I really like the whole Science Buddies thing that Banner and Stark have going on, but what Ruffalo does so well is play the jokes that Whedon loves to write without ever playing against the truth of Banner's ongoing nightmare.