Auteur Theory, emerging in the 1940s with the French New Wave movement amongst film critics and first largely introduced by Alexander Astruc (with Andre Bazin) and propounded in the 1950s periodical Cahiers du cinéma, centers on the concepts of "director-as-author" and "caméra-stylo" (camera-as-pen).
The theory holds that, just as a fiction author or a painter or sculptor has a persona, a style, a recognizable presence or voice, so has a director a recognizable persona that manifests in tension, shot style (e.g., reflection shots), thematic consistencies, perceptible world vision and other modes of persona projections that leave a traceable mark or signature upon the films that an individual filmmaker creates. Classic examples of auteur directors who are authors of their films and who use the camera-as-pen are Alfred Hitchcock and Charlie Chaplin. Modern directors who have earned the label auteur are, among others, Ron Howard and the brothers Ethan and Joel Coen. Their works, along with other past and present directors' works, display the personality behind the mind behind the film and exhibit a unique mark or signature.
Current opposition to the theory is embedded in the changes that have taken place in filmmaking since the early filmmaking years of the 1940s and 1950s. The objection says that filmmaking is such an intense effort in current times with so many gifted and exceptional specialists needed to oversee and perform every individual aspect of a film that it is no longer practicable to identify one single person as someone who imprints an identity upon a film and leaves such a strong mark that it amounts to a recognizable voice, signature, style or presence.
Well there is a foundational truth to this observation, two things tend to limit its applicability to the power a director does or does not have. One is the films themselves like Chariots of Fire and Oh Brother Where Art Thou and The Da Vinci Code that are produced today and the other is the persistence of the Academy Awards in acknowledging distinctive directors for leaving profound imprints on their films.
There are very few directors who are qualified to be the originators of films. Names that come to mind are Charlie Chaplin, Woody Allen, and Ingmar Bergman. John Huston once said that the three most important ingredients of a movie are story, story, and story. Huston had been a screenwriter and thus was able to turn out good adaptations on paper or to improve existing scripts on which to base his films. Alfred Hitchcock knew the importance of a good story and a good script. He went through hundreds of submissions before picking one project, and as far as I know he never did the writing himself.
A director is called a réalisateur in French. The word suggests that the director is supposed to take something that has been written and "realize" or "actualize" it in the cinematic medium.
Orson Welles was an auteur. His best film was indusputibly Citizen Kane. And that was because he had a wonderful screenplay written mainly by Herman Mankiewicz. Welles never produced anything comparable for the rest of his life. Most of his films are being forgotten.
One major argument against auteur theory is that the director is not always the writer of the film. While the director is responsible for determining all visual and audio parts of film-making, it may be less so in the digital age, where very small film crews can produce a feature film, with creative input is necessarily limited to a few people.
Another argument is that to focus on the director is to not only ignore the other aspects of filmmaking, but to trivialize them in favor of one dominating position, since the work of the director informs the work of the crew. Without a good cinematographer with a good understanding of light and focus, the film will be dark or washed out or blurry. The input of the film crew members facilitates the director's decisions.
Yet another argument is that to examine a film as if it were the sole product of the director is to place all praise and responsibility for filmatic choices, good and bad, on the director's shoulders. Producers often try to place blame on the director if the film does poorly.