Create compelling characters. They should be three-dimensional people, with positive and negative attributes, complex feelings, beliefs and thoughts. The characters must have needs, wants and goals.
Put regular people in extraordinary situations. Although there are some exceptions, most of the time people want to be able to relate to the characters. It is enthralling for people to see themselves in unusual circumstances.
Write believable dialogue. Dialogue must be natural from the character saying it, and relevant to the situation. Every word counts, and must reveal much about the character and the scene.
Have a well-thought plot, and subplot. The elements of a good story such as the goal of the character, the obstacles the character must overcome to achieve, the climax and the denouement must be included. At least one subplot including less important characters in a simpler story needs to be used.
Include action in the script. Action must also be believable and relevant to the scene. If no dialogue is happening, some sort of action should be.
Edit and revise many times. Have the script read aloud. Ask for outside opinions. Change the script until every second of the action and every word spoken works.