I'm a veteran of many writing classes and I've learned that there are two pieces of advice I can count on hearing on any class: "Write what you know" and "Write what scares you". Both seem simple on the surface but can be anything but simple when it comes to putting them into your work.

Big picture fears--the "what ifs" and abstract concepts--round out your characters' personalities. A character in a romantic comedy isn't likely to face a knife-wielding stranger in a dark alley, but knowing that your character (you) harbors a fear of strangers at night can add a nice spark of tension to a scene. How would your character react if the romantic interest burst out of an alley and ran towards her? What if he was carrying an injured kitten to the 24-hour veterinary clinic around the corner? Sure, that's a silly example, but it's worth running through the what ifs in your head (or on paper).

One of my fears is open water. I'm fine in a swimming pool, but large bodies of water like lakes, or--shudder--the ocean--make me blanch. In one of my stories, I not only placed a character on a small boat in a lake, but I set the scene at dusk so the character could watch the reassuring ring of land and trees fade into darkness. When I had to stop writing to get get up and turn on all the lights in the room, I knew I was on to something. Similarly, I applied this fear to another character living in a small oceanfront town. The local culture revolved around fishing, boating, and swimming, and he flat refused to take part in any of it. This gave me an immediate starting point for his personality (stubborn), the other characters' perceptions of him (stand off-ish, suspicious), and gave me a chance to poke around at his fear. I put him in situations where he could either confront or flee from water (fight or flight, remember?), and let the reasons for his fear infuse his interactions with the other characters. His fear of open water didn't drive the story, but it flavored the plot and heightened the emotional punch. Its like the baker's trick of adding a pinch of instant coffee to brownie batter. You won't taste "coffee", but it'll make the chocolate flavor pop.

Specific fears effect characters in the moment, while the broader fears round out their personalities. Project a fear or two onto your characters and dredge up all the aspects of the fear that make it so visceral or paralyzing to you. When you write with that depth of feeling and conviction, it adds depth to the reader's experience. If you happen to hit on a fear that your reader shares, your story will be unforgettable. Focus on the mundane or the profound, but always make a point to write what scares you. Your readers will thank you.