It took him the span of a full breath to process that the man - the angel - was asking for nothing from James in return. James recovered quickly; he had been too long an assassin to be shocked into stillness for more than a second or two. It didn't mean that he was unaware of the enormous gift Castiel had just bestowed. It was enough to restore any of the broken parts of his faith, simply knowing that there was still a God who sent angels.
"I will," he said. A pause. Then he dropped his head, not in subservience or embarrassment but in respect. "Thank you."
He turned from the edge of the rooftop and started across the City toward Ariel. Protect her. He'd do that.