Both Audrey Hepburn and Natalie Wood were dubbed after the fact. Each actress pre-recorded their own vocals and performed on camera to their own tracks. Marni came in after the fact and re-dubbed the singing.
Only Deborah Kerr knew in advance that she would be dubbed by Marni in The King and I, and the two worked closely together prior to recording, during, and after, to make sure it was believable ... which is probably why I think it's the most successful of Marni's dubbed roles. There are times where it's very hard to tell the difference even when you know a switch is coming. I also think that helped them plan the technical recording of it better.
When you hear a switch in My Fair Lady or West Side Story, some of it has to do with the physical recording ... the room ambience doesn't match, the volume isn't quite right, the echo (or lack of it) doesn't fit the words that come before it, etc. That doesn't help the illusion any.
All in all, I think Marni did a great job of matching Audrey, and there are only a few instances where I hear a noticeable switch.
Oh, as far as privacy goes, yes, Marni was sworn to secrecy. And it wasn't publicly acknowledged or discussed for many years, even if people "knew." It was never confirmed back then.
"Jaws is the Citizen Kane of movies."
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