Why it happens
- The line or its pacing changed after lip sync was done.
- The shot is in profile, head down, or the mouth is blocked.
- A two-shot doesn't specify who is speaking, so the model moves both mouths.
- Too much head movement makes mouth detail unstable.
How to fix it
Lock the script and the voice file first, then do lip sync, and don't change the words afterwards.
Frame dialogue shots face-on or near face-on, mouth unobstructed, with only slight head movement.
Split two-person dialogue into single close-ups, with each line spoken by one person.
Check every line for mouth open, mouth close, plosives and trailing sounds, not just the middle frames.
If the whole line is offset, shift timing in the edit; if the mouth is stiff, use a more frontal angle; if profile fails, change the angle instead of retrying endlessly.
Before and after
Fails
Two people chat in a car, both in profile, talking a lot.
Works
Lin Xia faces the camera, looking just off frame right, and says with restrained tension: “That's impossible…” Natural mouth movement, pauses matching the audio; only slight head reaction.
Prevent it next time：Plan dialogue as face-on single close-ups at the storyboard stage, and do lip sync only after the voice is locked.
Worked examples