There’s one shot in Alfred Hitchcock’s Psycho that you might have paused and pointed to as proof that Hitchcock showed a knife penetrating skin (or fake skin). You even see blood as the knife goes in. In reality it didn’t go in. “They put a little blood on the tip,” says filmmaker Alexandre O. Philippe, “and then put it against her belly button, and then shot it in reverse. That’s as close as it gets. But there’s never any actual special effect needed to show an actual wound. The body remains immaculate throughout the entire sequence.”
Incidentally, this was how Hitchcock bypassed the censors’ scissors. “It’s exactly what Hitchcock told them: No, you didn’t see this. You thought you did but you didn’t. I didn’t do the things you told me not to do. I was a good boy.”
Read more interesting facts about the shower scene in Psycho at the BFI’s 10 Things You (Probably) Never Knew About the Shower Scene in Psycho.