Well she was really the only other character in the film who he really cared for in-partI did think he seemed a little too handsy with her, but I don't know. It's technically Stark's imprint on his persona.
Her eyes go red when she mentioned "Stark" during the scene where Pietro explains to Ultron what happened to their parents. It was a nice touch.
She is the anti-Orlando Bloom.One thing I noticed about her is that she knows how to make very subtle facial expressions, which is a sign of a good actor. Like in some scenes you can see her demeanor literally change over the course of a few seconds.