Principal photography

Principal photography is that phase of producing an episode of Doctor Who in which the majority of the episode is filmed. It is the part of producing an episode in which actors are most heavily involved. It generally does not include any special effects, but usually does include practical effects. It also does not include things like pick-ups or re-shoots of completed scenes, both of which are ordered by the production team after principal photography has already "wrapped".

The amount of time taken to complete principal photography on an episode of Doctor Who has varied greatly over time, but it has generally taken longer to complete each episode over the years.

Doctor Who (1963)
In the Verity Lambert era, most episodes were completely taped in a few hours, largely because they were photographed sequentially as theatrical plays with minimal editing. As the editing of videotape became more affordable, Doctor Who was filmed out of sequence, and thus more time could be spent filming each scene. Location filming also became increasingly common. From the advent of significant location filming in the waning half of William Hartnell's involvement with the program, principal photography was split between "studio days' and "location days".  This meant that the total time needed to complete most episodes naturally increased for that reason alone.

There were three major patterns to the program's studio recording schedule. From An Unearthly Child to Spearhead from Space, the studio recording for stories would generally happen once a week. Material for only one episode would be completed during each day's recording. Until The Tenth Planet, this day was invariably Friday. From William Hartnell's last story until Jon Pertwee's first, studio recording was done each Thursday.

Barry Letts introduced a new principal phtography schedule beginning with Doctor Who and the Silurians. He decreed that the studio work for two episodes be completed on consecutive days each week. Thus, instead of it taking a month to complete the studio work for an entire four-part episode of Doctor Who, it would now only take two weeks. The innovation was borne of necessity. The outside contractor Barry Newbery had used to build the cave sets for Silurianss had delivered somewhat flimsy sets. Because they would be damaged by striking them each week, Letts came up with the solution of recording two studio days back-to-back, greatly reducing the number of times the sets would have to be taken down and put back up. The plan kept the cave sets from disintegrating, and the patten of once-a-week studio filming never returned to Doctor Who.

The Masque of Mandragora — or, to put it another way, Season 15, brought with it a significant, lasting change to the progress of principal photography. The idea of always recording on Fridays and Saturdays was abandoned in favor of a sliding schedule of two studio days one week being followed by three the next. This not only added a fifth day of studio recording, but also allowed for a whole month to be used, rather than excluding Sundays. , a fifth day of studio recording was added to the production schedule. Thus, many stories would have a two-day week of studio recording followed by a three-day week. Location filming would usually be done for 3 days sometime prior to the studio recording. In the case of Mandragora, this location filming occurred two weeks prior the start of studio recording, which meant that the total time elapsed for principal photography was five weeks. This schedule was retained for the rest of the original series, although stories with fewer episodes, such as Survival were completed in less time.

Doctor Who (1996)
The TV movie starring Paul McGann was the first "modern" production of Doctor Who. Filmed as a movie, it had a long and complicated shoot which lasted 9 weeks. As a one-off production, it did not establish any particular patterns which were later replicated by later productions or continue any legacies from the past. It undoubtedly established the record, however, for the longest principal photography of any single Doctor Who story. However, depending on one's views of what constitutes a "story", it would be bested by later, multi-episode arcs of the next incarnation of the program.

Doctor Who (2005)
An episode of Doctor Who during the Russell T Davies era of the program typically had a much longer principal photography schedule. However, it is difficult for those not on the production to speak intelligently about what the actual schedule was, since individual episodes were almost invariably described to the press as "behind schedule". Nevertheless, "Rose" was likely fairly close to meeting its schedule, and it took about 2 months of almost around-the-clock principal photography. Principal phtography was achieved in the RTD era through the use of recording blocks, typically comprised of three to four episodes, with photography from one episode sometimes overlapping with that of another. In the case of the Cyberman arc from Season 2, for instance, action from all four episodes was filmed jointly over the course of about three months, although material from Rise of the Cybermen/The Age of Steel generally took up the first six weeks, while Army of Ghosts/Doomsday was mostly recorded in the latter six weeks.