In the space of a second, the whole world turned.
"I'm scared, Sam."
Sam heard those words and in less than a second he knew what they meant and how he had to answer.
Dean hadn't simply said, 'I'm scared,' which would've meant manageable worry and apprehension and nothing Sam had to worry about. He hadn't said, 'I'm scared, Sammy,' which would've been Big Brother letting Little Brother know he was scared and that he wanted - needed - help; letting him know regretfully because Little Brothers should never know that Big Brothers could be scared.
He said, "I'm scared, Sam", and the whole world turned.
For the first time that Sam could remember, this was Brother asking Brother - brother, partner, protector, equal - for comfort and encouragement before he might literally give all.
All of his soul if not all of his life.
Sam wanted to immediately spout off every trick and tactic they'd already discussed and dismissed - holy fire wouldn't work, imprisoning Cain in the Bunker was logistically impossible, no binding spell existed strong enough to contain him - but he knew Dean wasn't looking for feeble options or false hope.
He just wanted some bit of something from Sam to take with him into the fight.
Sam had a lifetime of quotes of comfort that Dean had told him over and over again - 'this is nothing ~ I can handle this ~ don't be scared ~ you'll do fine ~ ' but they'd all be as untrue now as Sam had come to realize they were then.
The only fixed truth was not what they could do, but what they were. And Dean had given Sam all he'd needed and more of that when he faced his own battle against evil.
"I'm scared, Sam."
Sam nodded that he heard, that he understood. That he understood everything.
"I'm here, Dean. I won't leave you. I'll be right here."
The End