Bigby's seen plenty of unexpected transformations, so Alec's sudden shift has him stiffening in place, the fur on the scruff of his neck standing up as he bristles, reading that fighting stance on instinct before his conscious mind even catches up.

He doesn't quite relax when Alec comes back to reality, wariness in the set of his shoulders, but he consciously makes an effort to unfluff his fur a bit. Not like he's the first guy to have some kinda PTSD that Bigby's run into. "Don't worry 'bout it. Shit like that marks a person, Fable or no."