Zachariah was no fool. He knew exactly what his brother was doing. Castiel was manipulating his mistake into a more positive, respectable sort of light. As Zachariah wasn't supposed to be displaying any form of major distrust in his own sibling - yet - he would have been very much expected to swallow up Castiel's excuse with little to no complaint. Little was the more preferable option, as Zachariah did not want Castiel to escape this meeting without the heavy realization that Zachariah would personally turn him into a pile of dust if he didn't see to his job in the way that was expected of him. He may have had to play nice now, but he wouldn't always be required to do so. Castiel simply needed to get his job done; if that meant that Zachariah had to bend over and kiss his ass until he got where he needed to be, then so be it. That didn't necessarily meant that the kiss wouldn't come with a little bite, however. Hands still gripped together - albeit a little more tightly now that he had further reason to be annoyed - Zachariah shot Castiel a stiff smile. He was the manager of this operation. Castiel was the employee booking in on a new deal with a solid excuse in that he couldn't rage at him for delivering. What was he to do other than grit his teeth, fake a smile, and pat him on the back until he royally screwed up in an inexcusable manner?
"I question your approach, brother," Zachariah replied, the tone of his voice coming out awkward and uneven, "Sam Winchester and his pet demon are hardly worth focusing on. This little attempt at gaining their trust won't go very far with a demon thrown in the mix, will it? Can you honestly trust this demon to become a companion of yours rather than the driving force that keeps you from getting you where you need to be?" Zachariah raised his brows, before shaking his head critically and continuing onward. "I think, Castiel, you should stick to the original approach: focus on nothing and no one but Dean Winchester. He is our key - he's the one that we need on our side. Screw all the others."
They'd find another way to deal with Sam. That particular Winchester wasn't Castiel's responsibility and if he insisted on continuing on like an idiot in an attempt to do whatever it was that he was doing, then Zachariah would see to it that he spent some quality time upstairs. He didn't have to play nice. He could get what he wanted from Castiel in another way - he just didn't want to waste the time that was necessary in order to accomplish that feat.