One of the characters, a young man, is standing in a shadowy room with his back to the door when an unidentified character in a cloak and black hat sneaks in and slugs him into insensibility.
In order to achieve this, one of the necessary ingredients of the formula is a series of plausible situations with people that are real. When characters are unbelievable you never get suspense, only surprise.
The sergeant-major shook hands, and taking the proffered seat by the fire, watched contentedly while his host got out whiskey and tumblers and stood a small copper kettle on the fire.
a trinket thought to be a magical protection against evil
Her husband drew the talisman from pocket, and then all three burst into laughter as the sergeant-major, with a look of alarm on his face, caught him by the arm.