A scene changes when the actor begins listening instead of waiting for their line.

Waiting for your turn and listening look similar from outside and are entirely different from inside. An actor counting down to a cue delivers something technically correct and slightly dead, because nothing that just happened has affected them. The moment they start actually receiving what the other person says, the same words come out differently and the scene becomes alive.

Watch yourself in the next conversation for the point where you stop hearing and start preparing. When you notice it, drop your prepared sentence and respond to what was actually said. The reply will be worse and better at the same time.