Who is your story about? Why have you chosen this person? The answers to these key questions will go far toward helping you write your short script.
The first impulse of writers of short films is not to spend much time on the characters. The thinking is that because you have less time, you therefore need less characterization. This is totally wrong. In fact, your short film relies principally on character. Unlike in the long film, there is little time to deal with the complexity of relationships, but the viewers must feel that your main character has a complexity appropriate to the type of story you choose to tell. For example, in Incident at Owl Creek, it is true that we don’t have a profound understanding of all the dimensions of the principal character, but we fully understand his desire to live rather than to die. Similarly, we understand the two main characters in Two Men and a Wardrobe to be naive in a cynical world-but at least they believe in something! In both cases we understand and empathize with the characters in the context of their goals. Short films, therefore, do not tend to develop complex relationships between characters, but they do rely on complex characters to tell the story.