There are five major components to writing a screenplay: Character, Story, Structure, Voice, and Form. Now anyone can learn the technical aspects of screenplay form and fine tune the application of visual storytelling on the page: describing the shot, script economy, the art of the white space, etc... And all good screenwriting has a distinct voice: the way you describe the action, your style and word choice, the pulse of the page, it's rhythm, and how you connect with the audience.
The three remaining components, however, are the core of the script: Character, Story, and Structure. And even though writing a good screenplay is an almost impossible task, the essential elements of a good screenplay incorporates The Three I's: an INTERESTING Character in an INTRIGUING Story with an INTELLIGENT Structure.
Create INTERESTING Characters. A screenplay originates essentially two ways: (1) by creating an unforgettable character and developing a story for that character, or (2) coming up with a story first – usually high concept – and plugging in characters to compliment it. In the character approach, character is everything, hence the phrase "character driven". Think The Wrestler (2008) or Crazy Heart (2009) as examples. In the story approach, which is how almost all genre films are developed, the story alone cannot carry the movie. If we don't care about the characters, we don't care about the story.
Develop an INTRIGUING Story. Now, just creating an amazing character is never enough. There must also be a believable and memorable story situation for the character to operate in. And be smart about it. Just because it's a rom-com and we know everything works out in the end doesn't mean that you have paint-by-numbers. Surprise us; involve your audience with twists, mystery, revelation, and suspense. And be original. We love to see ordinary people acting bravely, so put the Everyman character in an extraordinary situation. Or if it's a story we've seen before, put it in an original world. Sometimes the world alone makes the story.
Apply an INTELLIGENT Structure. Your audience is pretty sophisticated when it comes to movies, and they definitely have some very specific expectations. And it's your job to deliver, but deliver in an intelligent way. This is especially important in more formulaic genre films: sci-fi, action, western, etc. So the first thing is to understand what kind of film you are writing and to what kind of audience. A rom-com audience has a different set of expectations as a horror one. And then it's all about organizing the story: outlining, acts, sequences, plot points, etc.