When Castiel first lays his hand on the Righteous man, raising him from Perdition, Castiel knows there's something different about him. His handprint is imprinted into the Winchester's shoulder, and he feels a strong connection with the man, even though they've never met or spoken. But Castiel has looked into the Winchester's soul, seen it laid bare; the love for his brother, and mother, and family, the self-loathing, the hopes and aspirations, the angel has seen everything Dean has to show, and he has never seen anything so beautiful and yet so flawed that he is awestruck by humanity. He tries to show Dean his true form, but he almost burns out the Winchester's eyes, almost renders him deaf, which would not have been good. So he assumes the body of Jimmy Novak, drawn to the vessel for reasons he cannot explain.
He calls to the Heavens that the Righteous Man has been saved, so loud that he thinks were he not an angel he would have permanently damaged his vocal cords.
He first goes to meet Dean, overestimating his landing and ending up on the roof of the barn he was summoned to. He had waited to be summoned, did not want to approach the Winchester and scare him (in hindsight, approaching him may have been less threatening than waiting). Castiel enters the barn, and the lights explode, even though he tries to contain his grace. He walks forward to the end of the barn, where the two men are cowering, and it's easy to tell which is Dean, even from a distance; Castiel is drawn to his soul like a moth to a flame, the purity calling to him, drawing him closer.
The bullets do not stop him as he walks forth, and eventually the two hunters lower their guns, exchanging worried glances. Dean picks up a knife from the table near him. 'Who are you?'
'I'm the one who gripped you tight and raised you from Perdition.' Castiel thinks it is a sufficient reply, feels that Dean already knows who he is, due to the bond they have through Cas' imprint, the ordeal Cas endured to find him and save him.
'Yeah, thanks for that,' and Dean doesn't sound grateful, lunging forward to stab Castiel in the chest with the knife in his hand. When it doesn't kill the angel, Dean's face falls, and Castiel smiles as he pulls the knife from his body, because people have done that before (it's a standard greeting, to be honest), and something about the Winchester humours him.
The other hunter tries to attack him from behind, but Cas has reflexes faster than lightning, and he stops the old hunter short, sending him to sleep before turning back to the Righteous Man.
'We need to talk, Dean. Alone.'
Dean's stare of disbelief sobers Castiel, and he no longer feels like smiling, and lets Dean surge forth to check on the other man.
'Your friend is alive,' Castiel assures Dean as he feels the other hunter glaring at him as he studies the spells on the paper.
'Who are you?'
'Castiel.'