The knife blow is delivered with enough force to crack the sternum.

Not problematic. Merely an interesting observation.

A more relevant observation is the fact that this meeting is not going especially well.

Perhaps, Castiel thinks, attempting to be friendly is a miscalculation. Dean, from what he has observed, does not necessarily respond best to overt gestures of friendship.

A business-like approach may be what is called for here.

And if there's one thing Castiel can be, it is business-like.

Without a flinch, and hardly a breaking of gaze, Castiel pulls the knife out of his chest and drops it to the ground.