Film directors are leaders that set out to inspire their cast into believing a vision. People may think of directors as a small part of a film's success, but they are actually the reason a film becomes successful.
One good example of this notion is analyzing James Cameron's Terminator films versus that of the new Terminator Salvation. Cameron has a vision in every one of his films because he knows how to use his abilities to direct the cast. He finds ways to maintain perfection, which aspiring filmmakers embrace as genius.
MC G could have directed a better Terminator film by asking more of his actors and removing scenes that boggled down the story. There were moments that moviegoers laughed, and that really ruined the Terminator franchise.
The TS film failed to produce fear, which had been a huge part of Terminator's success. Christian Bale's motivational speech at the end of Terminator Salvation represented one of those instances where a director failed to use leadership.
There is never a part in any of James Cameron's films that induce laughter, unless he meant for the scene to be comedic. Cameron is a serious filmmaker who has the talents to capture the illusion of storytelling. He always manages to successfully transfer his vision to the silver screen. Cameron continues to be a successful writer/director, yet again, with his new Avatar film.
While writers have authorship in constructing a film, directors provide the leadership required to carry out a vision.