Continuity is agreement of perspective, not duplication
Two sides of a conversation may have been filmed in the same room, but that does not make one room-tone file sufficient for the whole timeline. In a wide shot, the low air-conditioning hum, traffic outside the window, and short returns from the walls combine into the scale of one place. Move over a character’s shoulder and those proportions change. If the window has slipped out of frame, the exterior can retreat slightly to the side; a hard wall behind the other actor may return the voice more closely.
The shift should not be large enough to announce itself as an effect. A difference of one or two decibels, a change in the left-right weight of early reflections, or a little more high-frequency loss can be enough for the ear to accept a new seat. Replace the ambience completely at every cut and the location breaks apart. Good continuity does not freeze the background. It moves the listening point while preserving the rules of the same room.
Shot-reverse-shot has another invisible axis
The image has a 180-degree axis connecting two actors. Sound has an axis connecting a source to the position from which it is heard. If the window sits just outside frame right in A’s close-up, traffic can remain as a thin cue on the right. Turn to B and the same window should be heard in the opposite relationship. Letting that background clue turn is far less conspicuous, and often more persuasive, than panning every line toward a face.
If every sound flips with the camera, however, the entire room rotates like a set. Diffuse sources such as building ventilation can stay near the centre, while sources with a clear address—a window, corridor, or refrigerator—move with perspective. Separating what moves from what remains hides the edit while keeping the sense that two people occupy different seats in the same place.
Emotion appears in the character the background follows
The same spatial detail changes tension depending on whose shot makes it clearer. Let corridor footsteps approach only while we hold on the character waiting for an answer, and that person begins to sound aware of an exit even before anyone enters frame. In the close-up of the character making a decision, fold the exterior down and narrow the room slightly; attention moves inward without making the dialogue louder. Background does more than explain a location. It chooses what a character is listening to.
During the edit, mute dialogue first and watch the cuts through room tone alone. Check whether each shot differs so much that the place falls apart, or so little that the viewpoint disappears. Bring dialogue back and lower the changes until they stop pushing against the rhythm of speech. The audience may not remember the room tone. They will remember whose side they occupied as the scene crossed each cut, and that memory is built by these quiet movements.