This is a great article by Timothy Cooper about short films, but has a few great nuggest that could easily apply to writing as well:
Sharing here because by and large, the majority of the fan films we enounter here have more in common with short films than they do with features, and sometimes more than even actual broadcast hour-longs.
7 Simple Secrets for Making an Outstanding Short Film
Some pertinent points to consider:
The writer/director clearly demonstrates a personal connection to the story.
In Forever’s Gonna Start Tonight, the filmmaker has probably struggled with seeking acceptance, just like her lead. And we can all identify with the idea of having a crush on a teacher. The Crush’s screenwriter likely thought, What if I had taken that crush to the next level? For Please Say Something, writer/director/animator David O’Reilly clearly has a powerful and dark view of the struggles a relationship entails, and how two different people can feel as alien to each other as a cat and a mouse.
They tell a story of one character’s or couple’s journey—but no more.
These shorts don’t overreach. Remember: A short film is not a feature. It has little room for subplots, secondary characters, montages, or epic scope. It covers a single dilemma and resolves it in some way by the end. Too often, writers attempt to confront multiple dilemmas, introduce us to more than two main characters, or recreate a whole chunk of their feature film. A short is not the place for any of that. It’s the chance to attack one major scene or conflict—nothing more. Takeaway: Limit your reach. Utilize the “short” part of this medium by telling just one story and telling it well.
They feature memorable characters.
Film and TV are about characters; even the biggest special-effects-heavy movies are nothing without memorable leads. Gun battles, alien invasions, the apocalypse—they’re all really about the characters these events happen to. These short films demonstrate that the filmmakers can build characters that sear themselves in our mind, even in the space of just 10 or 15 minutes. Plot, twists, and story are all important, but without someone we can empathize with, your film will be forgotten. Takeaway: Give your characters strong personalities, quirks, wants, and goals. That way, we’ll care about them, and willingly follow them on their journey.
The entire article is worth reading and isn't all that long, but these points especially are ones I would point all fan film writers and producers to consider and pay attention to.