Star Trek II: The Wrath of Khan
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When Spock and Saavik speak to each other in Vulcan, Leonard Nimoy and Kirstie Alley actually spoke in English and then the sound people created the Vulcan words to match the movements of the actors' mouths, which Nimoy and Alley later overdubbed.
This was actress Kirstie Alley's first forray into movies.