The degree of control that a director exerts over a film varies greatly. Many directors are essentially subordinate to the studio, especially true during the "Golden Era" of Hollywood from the 1930s through the 1950s, when studios had stables of directors, actors and writers under contract.
Other directors bring a particular artistic vision to the pictures they make (see auteur theory). Their methods range from some who like to outline a general plot line and let the actors improvise dialogue (such as Robert Altman and Christopher Guest), to those who control every aspect, and demand that the actors and crew follow instructions precisely (such as Alfred Hitchcock and Stanley Kubrick). Some directors also write their own scripts (such as Paul Thomas Anderson and Quentin Tarantino), while others collaborate on screenplays with long-standing writing partners (such as Billy Wilder and his writing partner I.A.L. Diamond.) Finally, certain directors star, often in leading roles, in their films, from Orson Welles to Woody Allen to Barbra Streisand.
Directors often work closely with film producers, who are usually responsible for the non-artistic elements of the film, such as financing, contract negotiation and marketing. Directors will often take on some of the responsibilities of the producer for their films (e.g. Steven Spielberg), or work so closely with the producer that the distinction in their roles becomes blurred (as is the case with Joel and Ethan Coen).
The official film directors' union is the Directors Guild of America (DGA).