A frission of delicious, instinctive fear runs through him as Spike's face changes, reminding him of exactly who it is he's fighting here, just what this is all about. What might make it a little more fun than your average knock-about; what might even possibly manage to change his memories of this place for the better.
He grins as Spike backs up, fire flowing through his veins once more, and he goes right in after him, with his fists once he's got in close enough, backing him up against the wall at the T-junction at the end of the corridor. He does take a quick glance either way down the new corridor first, just to make sure their fun isn't going to be spoiled by both getting exterminated, but he only takes his eyes off Spike for an instant and then he's crowding in against Spike, one forearm across his collarbone to pin him to the wall and the other? Well, this is Jack so the other is cradling his head as he dives in for a kiss, fangs or no fangs.
He feels his lip tear against one of those razor-sharp teeth and tastes blood, and he swipes his tongue over the tear and feeds the blood generously to Spike.