A good screenplay has a protagonist, antagonist and conflict. Without these three elements, a screenplay can become boring. However, the antagonist does not have to be a person. Your main character could be struggling with life in a new city, an animal or even his own mind. Provided there is a character with which viewers can easily relate and conflict, the screenplay can develop into an interesting story. Always remember that originality comes from how you handle the set preferred structure, not in how your invented structure plays. Many readers will dump a confusing script unless you are an already well established writer.
Always invent your characters and place them in an environment instead of the other way around. Even the best told stories will fail if the characters seem more like objects than interesting people. Characters should change as the story progresses. It may be tempting to add humor with an unchanging character stuck in a changing environment, but the character must finally see the folly of her ways toward the end of the story for the humor to be effective.
Always research your characters. If one of your characters is a pirate, knowing about the lifestyle and tastes of the generic pirate can add a great deal of depth to your character. Do not be afraid to change a few things to create variety, but stick to things that people will generally associate with pirates. For example, a pirate could have almost every aspect of his life associated with how people expect to view pirates except for his collection of snow globes.
When writing your script, use capital letters for both the headings and the character names. While the dialogue should be centered on the page, the heading is a location descriptive instruction and should be on the left. Include camera angles where necessary, but avoid the use of emotional instructions as actors and directors prefer to interpret the script in their own way.
Many scripts are written in three act structural development. Most writers choose to do so by using the beginning of the story for back story and character introduction, the middle for conflict and the end for resolution. To accomplish this, they introduce the main character early and allow time for the reader to understand how the character thinks and reacts to different situations. You may introduce the antagonist immediately after the expository material or wait to introduce more secondary characters.
The conflict portion of this script is typically placed in the middle and should be the longest part of the script. While introductions and resolutions are interesting, the real material possibilities are almost always with the protagonist's conflict. Most screen writers will use their introduction material to continually set up the beginning of conflict. In this way, readers can see how a series of events lead to conflict rather than just introducing a problem with no reasoning behind it.
A script is rarely accepted without a great deal of fine tuning and rewrites. You may have a friend read the script or a producer may ask for a rewrite. Whatever the case, stay positive. You may receive a script full of discouraging red marks. However, make the changes and hope the second draft will be better. No matter how good of a writer you are, you will need to make changes for other people's vision. After the first rewrite, take a look at every word of your script and make sure it makes sense to the plot, characters and structure.