I love Mark Ruffalo’s choices when it comes to his body language as Bruce Banner, especially when it comes to his hands and arms.
When he’s first roped in, the hand-gestures are all anxiety: clasping his fingers, folding his hands over and over each other, as if holding himself physically back from doing something. Especially when surrounded by military-type organisations. Instinct tells him to smash them before they attack him. He’s restraining himself like no one’s business.
He’s closing himself down, holding himself in. Same goes for his arms tightly-folded over his chest. It’s a natural defensive posture, protecting ones self. Little by little, his arms unfold, but that’s when he starts fiddling to occupy his hands.
When you’ve seen the Hulk fight, you know his combat is almost always done with his fists. Bruce is keeping his hands occupied, and the means by which he does it could not be more symbolic: he’s twisting his glasses, the common symbol of intellect and learning, over and over between his fingers. His identity as Bruce is held - fragile and tangible - in the palm of his hands. They could be broken at any moment.
And then, when he shows up in NYC, look at his stance and body language. For the first time since he’s been around any of the Avengers, his arms are hanging loose. He’s relaxed. He’s at ease. He has no need for pretending or holding back anymore. He’s come to be useful, both as himself and as the Hulk, and he won’t hold back anymore.