Interruptions.
Never interrupt your enemy when he is making a mistake. Sometimes this quote (attributed to Napoleon) will apply very well to your fictional characters and you'll want them to keep doing whatever they're doing until the action or behaviour reaches a conclusion or leads to something new and perhaps surprising.
Sometimes, though, an interruption is exactly what you need, especially when you're stuck or your writing is failing to excite you. Stop for a moment and allow the unexpected to walk in. Have someone knock on the door, tap your character on the shoulder or stand up and ask an impossible-to-answer question. Being open to a certain amount of chaos can bring tired writing back to life, solve plot problems, add energy to the pacing and add layers to your characters.
The characters below are all in the middle of doing something. Write about what they're doing and then interrupt them.