For me, it is the scene where David and Amy are playing in that dusty, cob-webbed room, filled with junk, Amy telling David that she really can hear Quentin on that disconnected Victorian phone. David teases her, but when he speaks into it, intending to prove that she's just a silly girl who doesn't know the difference between make-believe and reality, that expression of terror-ridden shock and surprise on his face, heightened by the quick camera-pan-in-close-shot, caused by him hearing a disembodied voice we can't hear, was simply chilling. Those two kids were absolutely incredible in their acting ability.
Gerard