If your shot of the presenter is a close-up or even MCU, then there is little hope of merging in a mouth shot unless you really have plenty of shares with Lady Luck. A viewer watches eyes and mouth, so anything less than perfect would be distracting and really stuff up your video flow. If the dialogue is about the product, it would not be distracting for a cut away to that product… we all want to see what is being discussed.
Anyway, that’s my two cent’s worth.
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