Dialogue is where generated films most often break the spell. Three decisions keep it believable.
Choose the voice before the face
Pick the voice in the character bible so every line of that character uses it. Switching voices mid-film is the audio equivalent of a face drifting.
Short lines, clear consonants
Lip sync works on visible mouth shapes. Lines with hard consonants and pauses sync better than long murmured sentences. Rewrite for the mouth, not just for the ear.
Which sync tier
The standard tier syncs a clip to generated speech and is fine for medium and wide shots. The pro tier takes your own audio and is worth it for close-ups, where the viewer watches the mouth.
Bring your own recording
If you have an actor, record clean audio and upload it as an asset. Pro sync uses it directly, and the performance is yours rather than synthesized.