A girl agrees to be given drugs against pain and panic in order to be mutilated alive- Maybe as performance or for a photoshoot- And then killed before their effects fade.
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A producer or something of the sort meets with a caretaker raising artificially-grown and/or orphaned girls to be actresses. Upon hearing that an authentic death scene may be in order(or maybe inquiring about that), she tells him to pick any girl that's there with them for an immediate performance of a certain climatic scene- They have been trained so well that they are all ready for that at any moment.
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A girl training with swords as a hobby discovers a technique for self-decapitation and practices in it with a specially weighted but shorter substitute. Perhaps she keeps it a secret.
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Addendum to >>14275
One reason penetrative massage has gotten so popular is the invention of a self-training tool, a sensor-filled phallus that connects to a computer with accompanying software to guide the aspirant in the correct movements, simulating the nervous responses expected. It is sold at a loss by organizations employing such masseuses as an investment, so it's pretty cheap- Enough for young girls to secretly buy one to train with. The videos might not be age-restricted either, being for educational purposes.
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A girl signs up to be killed for a movie, assuming she'll be in a certain very important scene in the story along with a famous and highly attractive actor. Instead she's sent to the second unit for a quick and rather inconsequential death.
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A big-name actress is going to do an authentic death scene. But to make sure it goes as planned, a stand-in is required: Someone with the same build whose body will react the same way, to do it first and show if adjustments are needed.