Filming "back-to-back" refers to the practice of filming two or more movies or episodes as one cinema or television production, reducing costs and time.
Contents
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.
Rationale
In modern filmmaking, the entire cast and crew for each film is assembled from scratch for each project, and each of them is laid off and sent home as soon as they complete their assigned tasks. Almost all participants in the industry are freelancers, who move easily from one project to the next and don't have much loyalty to any particular studio, as long as they get paid.
This differs from the old studio system in which studios carried large numbers of cast and crew on their payrolls under long-term contracts. 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 (each with its own unique staff) for every single film.
The advantage of the latter system is that film studios no longer have to bother either with paying people who aren't involved in a current film production, or with green-lighting films very frequently so as to get the most out of sunk costs in their human resources. However, this also means that 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 (and ultimately their investors or backers) 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 can't get that person, they have to scramble to locate a satisfactory substitute. All successful directors and producers have certain favorite cast and crew members with whom they prefer to work, but that's of no help to the studio if that perfect character actor, costume designer, or music composer is already fully booked.
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 (before anyone dies, retires, or does something else that allows them to command an even higher fee) 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.