In the Daily Writer, Fred White includes a list of five questions that can really help the development of your fiction, whether long or short. These questions concerns the motives of your characters.
1. What do they want?
2. Why do they want it so badly?
3. How do they intend to get it?
4. What stands in their way?
5. How will they overcome these obstacles?
The answers to these questions drive all great fiction, in subtle or more blatant ways. As an editor, I often find that beginning writers spend too much time obsessing about plot twists and too little time focussed on the motivation of the main and secondary characters. Plot doesn't exactly take care of itself if you focus on character motivation; you still need to devise interesting incidents and coincidences and turns in the arc of the story. But without the deep, underlying motivations for their actions in the story, we won't much care.
During the writing of my first novel, Witnessing the Creation, it didn't become really clear to me what the main character was after until about a third of the way through the first draft. I knew he was dissatisfied and yearning but exactly why and what he was after didn't come clear until then. I don't know that one needs to do the character question answers before starting a piece of fiction, but if you can do it early on, it will help guide everything from description to dialog and become a main thread of the story that readers can rely on.