It's a testament to Dean's arrogance and self absorption that he should think that he's getting the silent treatment, that Castiel's quietness has anything at all to do with him. He's not speaking because he does not want to speak, because he has nothing to say that would not end in argument, and he is weary of arguing. This creature is his imprisoner, plain and simple, there's no call to be friendly with him, no need for smalltalk, and he's far too chafed to care about him at all. To be perfectly honest, Castiel might be occasionally wrathful, but he is not vengeful, he's not here to make Dean suffer (much the opposite, if you ask him, but he's not in the mood to chat, either. He's not trying to get to Dean. Honestly, he'd much rather simply be left alone.

But apparently his imprisonment isn't torment enough, he's going to be forced to socialize as well, with the beast that abused his father, and seems to enjoy patronizing him for sport. He can't say he understands why Dean is bothering to make conversation, but Castiel isn't petty enough simply to ignore him for the sake of ignoring him. So he presses his lips together, and nods sharply while he walks. The mention of a library, however, certainly has him perking up just a little.

".. a library? Where?"