Sam Winchester understands what they experienced in purgatory most in the little things. The little things are what reveal the most, he knows. Like when they are in motels, and Dean relishes in the comfort of even the dingiest of beds. It’s in his haughty laugh and his stupid jokes- it’s in his insistence that the crap bed beneath him just has to be a temperpedic that Sam can tell he spent days struggling to rest in the most painful of places. It’s revealed to him when Dean returns from a quick run to the store or from a hustle at the nearest bar, Sam watches Castiel’s face brighten in the reassurance that Dean has returned to their side yet again, the younger brother knows there were too many moments of doubt. That the angel whole-heartily feared that Dean would not return from whatever doom he walked into far too many times for his body not to commit the slump of the shoulders, the sigh of relief, the warmth of his features to muscle-memory.
They don’t talk about it much. Not to him. They whisper things to one another when they are alone, sometimes, outside by the Impala, or when resting on dew-covered grass and downing down burgers, or when sitting across from one another, mistakingly certain that Sam is asleep in the bed beside them. He cannot hear their words, and when he can their vague conversations make little sense; but he sees it in the way their bodies curl towards one another, in how they lean into a defensive shell when they are beside one another in the silence. He sees it in how Castiel blinks on cue with his brother. He sees it in how Dean tenses when Castiel flinches. Sam took sociology in college- he knows what trauma does. He knows that catastrophes reveal the innermost and that tragedy repairs even the most severed of relationships.
When they hunt, he understands the most. In the way they move together, even when they don’t try to. In the way that they are conscious of even the slightest breath the other misses. Sometimes Sam feels transparent, in those moments, almost forgotten. Only sometimes, and the feeling is fleeting. But he is always, always, amazed. Amazed in how Dean can differentiate the flashing blur of a Wendigo and his angel with just a moment’s glance. Amazed in how Castiel can tell when it is the hunter or a predator walking towards them on the sole basis of the sounds of their footsteps.
When they are hunting, when they are preoccupied, Castiel and Dean reveal to Sam everything he’d ever need to know. They don’t need to speak, and Sam does not need to ask. Every question, sooner or later, is answered in the days that follow their much-awaited return.