Boring dialogue is one of a script's elements that will stop a reader. He'll put down the script, possibly never to pick it up again. Two exercises of writing in character voice can help you get in contact with the speech rhythms and vocabulary of a character.
If you read scripts by other writers, you will often find the same flaws that industry readers find. One of the shortcomings is flat dialogue.
Write a monologue in character voice. This will help you learn a character's insights, thoughts and feelings. Pick a situation. What are her hopes? His fears? What does she love? Hate? The character is speaking to himself; you must get "inside" the character to write in his or her voice.
Write a speech given by the character. Now the character is addressing other people. How are his speech patterns different from the monologue? What vocabulary is the same? Which words are more formal? Visualize the person or persons to whom the character speaks. How does she convince them of her feelings? How does he move them to action? You will learn how your character uses words to affect the thoughts and actions of others. This is definitely the core of dialogue.
If you do these two exercises for each of your main characters, you will develop speech patterns rhythms and vocabulary that are essentially theirs. Use this character-unique expression to differentiate dialogue in your script. Your characters and your story will be richer and more intriguing.