Even if they do have to voice over - they have to exactly match movement with sound - not easy, and it can't fix any goofs, as it would be obvious when the face/voice are out of sync.
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Beware of dragons for you are crunchy and go well with ketchup
Dr. Horribles sing along blog
You people all have to learn
This world is going to burn
Burn
(yeah, it’s two r’s. H, O, R, R, yeah right.)
Burn