From Disney Pixar Toy Story

Tension must be intricately laid throughout a story, not just appearing in moments of distress or while a character strives for a goal. A great trick for inducing a constant underlying tension is to portray your protagonist as uneasy, uncomfortable, or at odds in each scene. She can be hot, cold, bored, nervous, lonely, hungry, tired, aching, or craving a drink or smoke. The point is, she is rarely at ease, rarely happy, and rarely comfortable.

As a writer, you’re constantly looking for opportunities to make your character feel out of sync with her surroundings. You send a rookie cop to a grisly crime scene. You force an introvert to attend a party. Because you’re striving to use tension as an underlying factor in every scene, rarely feature your character alone in a scene. The reason is simple: A character alone can be static. If you’re tempted to create these protagonist-alone scenes, it’s helpful to imagine your character on a stage alone and immobile. Sit in the audience and observe her, and ask yourself what  you can add to make the scene sizzle with tension.

If you’re forced to create scenes where a character is alone, find ways to introduce tension. Perhaps while alone the protagonist recalls a painful memory, and thus the scene segues into a flashback. Or, she could be deliberating over difficult choices or thinking back to the previous events in the story. In a transitional scene where the character is driving or walking to the next locale, she can be dreading what is about to happen, fighting traffic or a downpour, or in some way interacting with the environment.~ from Between the Lines: master the subtle elements of fiction writing