Yeah, they wouldn't credit the actor. Just like Jennifer Waldman is Dorothy, and she isn't credited for that..
It's the same thing for the witch transformation in "Into the Woods." The real witch slips offstage during the scene and a body-double comes on while she is changing into the "beautiful" witch. When I saw the revival, the recording was so bad that it was OBVIOUS the double was lip-synching. I think I remember the same thing with the Boq transformation, only the recording wasn't as bad -- you can just tell that the sound is different for Boq's speaking after the transformation and Nessa's LIVE speaking.
I think it's clearly another actor at that moment. Just like the Wizard in the bginning. The person who plays the Wizard is singing, but it's another actor giving the mother the potion.
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