"Horror Writers are specialists in the worst case scenario." - Richard Laymon
I love this quote from the late Richard Laymon. I've blogged in the past about using "what if" as a question for generating plot ideas.
If you write horror, perhaps an even better question is: "What's the worst that could happen?"
Off the top of my head, I"ll use a doctor's visit as an example.
You're sitting in a doctor's waiting room. You're seeing a new doctor for the first time. You start to notice the office looks a little run down. There are no other patients in the waiting room. The rug is dirty. You notice cobwebs in the corner. Uneasy, you get up to leave when the nurse calls you into the treatment area.
You consider leaving, but you really need to see someone for the medical problem you're having. Maybe it's a suspicious growth. That part can be worked into the story. It's just your imagination making things worse.
As you walk down the hall, you feel a pinch in your arm. Something is injected and your limbs grow heavy. You try and fight it, but next thing you know, you're on the ground. A ghoulish doctor bends down and smiles a yellow-toothed grin at you.
When you awake, you have been stripped to your underwear and strapped to a table. The doctor, clad in surgical garb, approaches with a bone saw.
Your job as a writer is to make things worse from here and have your character fight his way out of the predicament you've put him in.
Keep asking, how can it get worse? And how can my character fight his or her way out of this?