Alistair's head turns at the sound of the roar. It -- doesn't seem like a frightened sort of sound? Maybe?

"Stay here," he murmurs to Kieran anyway, and rests his hand on the pommel of his sword as he moves next to the door.

Which means his ear's right next to the wood when Cullen knocks. Alistair jumps about a foot; about a quarter of a second later, he yanks open the door.