For me I like it when, how faster they breathe while doing 'that' and how good they adjust the distance to the mic to make the kiss sound immersive(◡ ω ◡)
How they match their 'movement' or breathing with the SE lol Sometimes the voice actor is too fast, too slow, or acts like he never even heard the SE at all. I really love it when he sync up with the SE because it makes the drama cd feel so real.
The way he finish and lean in for a kiss always melts me too lol