Despite the elevation of the film director since the auteur theory was developed in France and embraced elsewhere, films are not always - if ever - the vision of one person. And sometimes that's inarguably the case, when one or more other, often uncredited, directors make significant contributions.
In the old studio era of Hollywood, directors were usually subservient to producers, the latter considered a more important job. It was far from uncommon for films to have multiple directors, most of whom were not credited: they, like other studio employees, pitched in where they were needed.
Even after the studio system had lost much of its power, major directors sometimes helped each other out without credit. For the biblical film The Greatest Story Ever Told (1965) David Lean (Lawrence of Arabia) and Jean Negulesco (How to Marry a Millionaire) filmed scenes for co-writer and director George Stevens during the long shooting schedule.
Metro-Goldwyn-Mayer's Victor Fleming - who had a reputation as a no-nonsense sort who could handle difficult productions - was the director of record for two of the biggest films of 1939. On The Wizard of Oz, he was preceded by Richard Thorpe (none of whose footage seems to have been used, as producer Mervyn LeRoy thought it lacked the requisite sense of wonder) and George Cukor, who spent a few days reworking things: his major contribution was to get rid of Judy Garland's blonde wig and heavy makeup and to encourage her to act naturally.
Fleming directed most of Oz but was replaced by King Vidor, who directed some retakes as well as the Kansas scenes, but didn't take credit. Clark Gable had requested his friend Fleming be reassigned to Gone With the Wind, replacing Cukor.
Fleming directed about half of GWTW, including most of the scenes with Gable: Sam Wood replaced the exhausted Fleming for a few weeks and portions were directed by others including production designer William Cameron Menzies and producer David Selznick as well as second-unit directors (a job usually entailing work such as action scenes and establishing shots of locations).
Speaking of second-unit directors, Ben-Hur (1959) won William Wyler his third best director Oscar but Andrew Marton and Yakima Canutt were in charge of most of what became the best-known sequence in the film, the chariot race. The film won 11 Oscars (a record subsequently equalled but never surpassed) and, like GWTW, was a huge hit.
Another production with multiple directors turned out less well. Cukor and LeRoy were two of four directors who worked on the romantic melodrama Desire Me (1947). None of them wanted their names associated with the film, so it was released with no director credited on screen. Had it been made between 1968 and 2000, the directorial credit might have gone to Allen (or Alan) Smithee. This name was the official Directors Guild pseudonym devised for when a director did not have effective control of a film and wanted to disown it. Some directors have used it on re-edits of their films for TV or airlines.
The Smithee name was first used on Death of a Gunfighter, where neither the original director, Robert Totten, nor his replacement, Don Siegel, wanted credit: apparently star Richard Widmark was very much in charge. Smithee "directed" many films until the 1997 satire Burn Hollywood Burn: An Alan Smithee Film. In it, Eric Idle plays a director unhappy with his latest movie whose name really is Alan Smithee. In what was either a supreme irony or a desperate publicity gimmick, Burn Hollywood Burn's director, Arthur Hiller, reputedly resentful at producer-writer Joe Eszterhas's interference, had his name replaced by - you guessed it - Alan Smithee. The critically reviled flop brought such publicity to the Smithee name that it was eventually retired.
Superman director Richard Donner got on so badly with the producers that another director, Richard Lester, was brought in as a go-between. Donner had already directed a lot of Superman II but was replaced on it by Lester, who had to refilm some segments in order to meet the minimum amount to be credited as director. Donner declined to have a shared credit on screen. Many years later, "The Richard Donner Cut" was released, with as much of Donner's footage as possible restored to approximate his version of the film.
The Thing from Another World (1951) is credited to Christian Nyby but some of the actors contended that much or all of the film was directed by its producer, veteran director Howard Hawks. Nyby - who had edited some of Hawks' films - was given the directorial credit as a favour, the story goes, so he could join the Director's Guild. Nyby himself denied that Hawks directed the film but acknowledged the more experienced filmmaker's style influenced him.
Again, accounts vary but the producer and co-writer of Poltergeist (1982), Steven Spielberg, seems to have directed some or all of the film, which is credited to Tobe Hooper. And George P. Cosmatos seems, for whatever reason, to have been usurped a few times. He came in as a replacement for the fired director of Tombstone (1993) after a few weeks of filming but star Kurt Russell, according to himself and co-star Val Kilmer, took on much of the directorial role, planning shot lists for Cosmatos to follow. Sylvester Stallone is reported to have similarly dominated Cosmatos on Rambo: First Blood Part II and Cobra.
And there was fierce disagreement between The Room's producer, writer, star and credited director Tommy Wiseau and script supervisor Sandy Schklair - the latter claims he, not Wiseau, directed the film and at least one actor has been quoted as supporting this. Why he would want credit for the notoriously bad film is another matter.
As for screenwriters not receiving credit, well, that's another story - and an even messier one.