Thor, on the other hand, is not tense. He looks relaxed and easily ready for action, and very nearly cheerful, as if this were the start of a pick-up football game or something.

You know, if pick-up football games usually entailed light armor. (Bare arms, no cape, only a sleeveless flexible breastplate and vambraces -- this is Thor's equivalent of workout clothes.)

Mjölnir lies on the ground some yards away. It'll come to his hand if needed, of course, but if not -- well, X did ask him to please try not to wound her too badly while Bruce was around, so as not to upset him. Bare hands are a lot easier to avoid inflicting bloody harm with than a giant magic hammer of magic stone.