This is a video of foley artist Matt Davies discussing and demonstrating the art of creating sound effects for zombie movies (the term foley artist was named after early sound effects pioneer Jack Foley -- that's true by the way, I'm not just pulling your chain to make it sound like there's a ghost in the attic). I remember when I was a kid making zombie movies with my neighborhood friends I used the exact same raw chicken technique for gut squishing, then my mom came home and was all, "Well there goes chicken pot pies for dinner!" It's cool though we had a frozen pizza instead so it was still a win-win as far as I'm concerned.
Keep going for the video.
Thanks to Josie, who agrees movies sell lies and an actual skull getting crushed probably doesn't sound anything like smashing a green pepper in your hand.