Filming "back-to-back" is the practice of filming two or more movies as one production, reducing costs and time. Trilogies are common in the film industry, particularly in science fiction, fantasy, action, horror, thriller, and adventure genres. Production companies may choose, if the first film is a financial success, to green-light a second and a third film at the same time and film them back-to-back. In a case where a lengthy novel is split into multiple installments for its film adaptation, those installments will usually be filmed back-to-back.
In modern filmmaking, employment is now project-based, transitory, and "based on a film not a firm." Almost all participants in the industry are freelancers, who move easily from one project to the next and do not have much loyalty to any particular studio, as long as they get paid. This differs from the old studio system, a form of mass production in which a studio owned all the means of production and carried large numbers of cast and crew on its payroll under long-term contracts. Under the old system, "a producer had a commitment to make six to eight films per year with a fairly identifiable staff." Under the new system which replaced it after 1955, filmmaking became a "short-term film-by-film arrangement" in which a producer is expected to assemble an entirely new cast and crew for each project, and rent the means of production from contractors only as needed. To borrow a factory analogy, studios transitioned from using a single assembly line with an integrated staff to continuously churn out one film after another to building and disassembling separate assembly lines for every single film. The advantage of the current system is that film studios no longer have to bother either with paying people who are not involved in a current film production, or with green-lighting films very frequently so as to efficiently exploit sunk costs in their human resources. Studios shifted from a emphasis on "speed in production" to "more cooperative pre-shooting planning." But now, when they want a particular person for a film, that person may be unavailable because they are already committed to another film for another production company for that particular time slot. In turn, for every single film, studios end up bearing massive transaction costs because they not only have to get the right person at the right price, but at the right time, and if they cannot get that person, they have to scramble to locate a satisfactory substitute. All successful directors and producers have certain favorite cast and crew members whom they prefer to work with, but that is of no help to the studio if that perfect character actor, costume designer, or music composer is already fully booked. Compared to the previous system, directors and stars spend a much "larger part of their time negotiating each new film deal." Therefore, if a film does well at the box office and appears to have established a winning formula with a particular cast, crew and storyline, one way to minimize these transaction costs on sequels is to reassemble as much of the team as soon as possible and sign them to a single production that will be edited, released, and promoted as multiple films. This also minimizes the problem of stars visibly aging between sequels that do not have significant time gaps written in between them. The pioneer of modern back-to-back filmmaking was producer Alexander Salkind, who decided during the filming of The Three Musketeers to split the project in two; the second film was released as The Four Musketeers. The cast was quite unhappy to be informed after the fact they had been working on two films, not one. As a result, the Screen Actors Guild introduced the "Salkind clause," which specifies that actors will be paid for each film they make. Salkind and his son Ilya went on to produce Superman and Superman II back to back.
Superman and Superman II were filmed simultaneously in 1977 to be a two-part epic. However, due to off-screen difficulties between the producers and director Richard Donner, production on the sequel was stopped in order to finish the first film for a December 1978 release. Filming on Superman II resumed in 1979 with a new director, and was released in Australia in December 1980 and in the UK and US in 1981.
After the success of the 1985 film Back to the Future, the two sequels Part II and Part III were in production from February 1989 until January 1990 with only a three-week break in principal photography between films and some of the filming of the third film overlapping with the second. The two films were released six months apart, in November 1989 and May 1990 respectively.