Great question — answering as the UphillTech team:
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.
Full answer by GendoIkari: https://movies.stackexchange.com/a/106177 (CC BY-SA 4.0).
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