"I tire of many things," he answers, with a light shrug of his shoulders. "And yet I continue to do them. Mayhap when it is no longer worth the thought spared, I will cease."

But he doesn't sound particularly bored right now, nor tired. There's a bit less of his usual detached demeanor in the tone of his voice, in the subtly straighter posture... and if that conveniently makes Astarion's life a little more difficult with their height difference, ah, well-- that just can't be helped. It certainly doesn't displease him that Astarion took the lead anyway, he won't fault the boldness of the move.

He won't fault it, but he will disrupt it, just to see how Astarion handles the situation. Emet-Selch waits for a step that necessitates moving a hand away, a shift in position, and finally makes his move: a graceful step in and past, abruptly reversing their positions, with a smooth motion of one arm aiming to dislodge and reposition the hand resting on the small of his back so that he can replace his at Astarion's, and thus steal the lead.

It's meant to be a quick thing, taking advantage of an opening; he's curious, after all, whether Astarion can or will deflect.