Years ago, I was in a broadcasting class, and the teacher asked that question. It had been done on early radio show when there were no CGI effects, and they were limited to sound and imagination only. I’m thinking of this question now because I’m writing a scene in Miasma where Keymas, the main character, witnesses a man struck by magic who turns into a monster. It’s not a cool thing like turning into a werewolf, but more like something the Nazis would do as experimentation.
The teacher never gave the entire class the answer — only to one person who guessed tearing up a Styrofoam cup. This is one place where I actually want to stay away from the visual side and play up on the other senses! It might actually be scarier!