What is the secret formula for scaring someone [No cheap scares!]

[Rapture]

In my current custom story I'm making very frequent use of grunts that don't alert when they appear. Makes it very handy to have them sneak up on the player

I must ask, how do you script it?

You don't script it, its a property of the enemy. Go into the "Model Editor" and open up the enemy you want. In the menu, go to > Settings > User defined Variables. Inside are the properties for the enemy. Scattered around are things like sounds or music to be played on activation and such. Just delete them and save the monster out.

Make sure you make a new copy of the enemy before doing this.