Outline the story you will tell in your script. If you have a particular actor or actress in mind, think about the types of roles he or she generally plays and tailor your material to fit that person.
Leave room for a director to do his job. Write your script with strong, present-tense action and good dialogue. Don't worry about writing in transitions. Don't write shots and camera angles. Leave this for the director. A director will most likely ignore anything you put in the script that tells him how to shoot the film.
Leave room for actors to act. Your job is to tell a story on paper. A director tells the story visually. Actors bring the characters in the story to life. Very rarely do you need to notate in a parenthetical that an actor should smile coyly or any other acting direction. An actor brings his or her own style to a role. Actors are trained to know how to play a scene and will most likely disregard any parenthetical acting direction you include in your script.
Write a solid story with a beginning, middle and end. Tailor the script around an actor or actress you think would be right for a role in the film, and then let everyone else do their jobs. This is the type of collaborative effort that makes for a good script.