When Lex falls through the ceiling in Steven Spielberg's 1993 classic Jurassic Park, we clearly hear her screaming. However, as we hear her screaming, we do not see her mouth visually moving. How does that work? Am I missing something?
The actual person you see falling there is a stuntwoman; you can tell if you look closely that the actress's head doesn't quite look right; that's because it is Ariana Richards's head on the stuntwoman's body, using CG.
Sound mixing is a big part of making movies; the sound you hear throughout a movie is often not filmed as part of the same video that you are seeing. Here; the mixing of Lexi screaming is just not perfect with the video. However, you really can't see her mouth during the screams itself, so I don't think it is obvious from the sound alone that Lexi is not screaming on video at the right time.