Cas turned his face away, his shoulders hunching as Sam spoke. He knew Sam was in pain. He knew he was exhausted. His mind kept trying to wander - this was too much, too important, and he had a habit of messing things like that up. He was terrified in a way he hadn't really been since he left Sam and Dean behind months ago. But... here, now, Sam needed him. He knew that. There weren't any other options. Sam needed help – the kind of help that only an angel could give, and Cas was the only angel around right now.
Start small, perhaps? Start with looking Sam in the eye. He turned back to face his friend. Castiel didn’t require sleep himself, but he knew the dark bruising under Sam’s eyes, sunken into his too-pale face, were not signs of health, and he winced in sympathy.
“I know you want – you need – my help, Sam. Perhaps… Let me… Let me just have a look first…” Cas did not wait for permission, but raised a hand to Sam’s face, resting warm fingertips against his temple. Then he reached out with his Grace, the warmth glowing from his hands and flashing white-hot in his eyes, to feel out all of Sam at once – body, mind, and soul.