Dean Winchester has influenced Castiel. This much is obvious.
What isn't clear is the extent of his influence. Enter Zachariah, to watch and learn and figure that out.
And he's been watching them very carefully. He knows that while much of the plan is secure, there are still one or two uncertainties. So long as Sam Winchester is blinded by his hatred of Lilith, nothing can go wrong. So long as Dean Winchester believes Heaven will allow him to save his brother, nothing can go wrong. So long as Castiel doesn't tell Dean the truth, nothing can go wrong.
Too many variables for his taste.
At the beginning of all this, Castiel was the sturdiest pawn in the game, voted least likely to be affected by a human. And look at him now - a few months in Dean's presence, and he starts going around directly disobeying orders! Zachariah had never seen anything like it.
He's been assured that it won't happen again, but Zachariah didn't get to where he is now by accepting everything he's told as the truth. So even now, petting Castiel's hair and throwing a knowing smile at Dean, he's keeping an eye on them.
Castiel seems subservient enough. This new vessel is making it easier for her to hide her thoughts from Zachariah, but he can see enough that he's not worried.
Dean is much easier to read - he's just pissed. At Castiel, at Heaven, at his brother, at himself, nothing new here.
There's no reason not to let Dean shout at Castiel while they wait for Sam to get a move on, so Zachariah pops out for a minute to watch the beginning of the end. The excitement's getting to him, the anticipatory tingle in his spine an almost thrilling sensation as he watches Sam storm into the church, bloodthirsty and ruthless.
He thinks he might understand now why humans watch movies. This is almost fun.
But as Sam starts throwing open doors with his mind, Zachariah feels a tickle. An itch, really. Someone's painting a blood sigil on the wall of the - oh no. Not now, Castiel.
But no, it is now, she would decide to screw the pooch right at the end. She slams her skinny little hand against the wall, sending him free-falling through space-time, and between the panic, embarrassment, and anger, he reflects.
They'd really had no idea.