Though the director is usually considered a film's "author" because it is the director's vision that makes a story come to life, the story itself is the work of a screenwriter or writers.
The Work of a Screenwriter
A screenwriter either develops a script from an original idea (their own or someone else's) or adapts an existing body of work, such as a book, play, or even a previous movie, for filming. Many screenwriters are hired to work on second or third versions of a script as rewriters.
Screenwriters may first develop a treatment of the story idea, which is a synopsis of the film with a few scenes written, but without dialogue. Then a "shooting script" is written, which is what will be used during filming. Generally one page of script equals one minute of filming, so many scripts must undergo numerous revisions to fit into the allocated budget of the film. The shooting script includes dialogue and instructions about lighting, camera angles, and shots. The screenwriter works closely with the director and producer, and will often incorporate changes in the shooting script to reflect their requests.
Screenwriters on assignment must not only be creative, but also able to collaborate with others, and write and rewrite multiple versions of a script, often under pressure to meet deadlines.
Training & Education
Though not required, many screenwriters hold at least a bachelor's degree in film or writing.
Understanding other areas of film production, such as direction, budgets, editing, and cinematography, are all helpful in writing a script that is commercially successful.