Pre-production is the preparation made before filming of a television show or movie begins. This often entails writing, casting, design, etc, and is considered completed upon the production of the actual content.
Pre-production begins with the greenlighting of a project. Once it is approved for production, preparations are made to finalise the approved draft via financing and the hiring of staff. The latter mainly consists of a director, cinematographer, and principal cast members, the latter of whom are usually approved from an audition (in the case of a new role or a necessary substitution of an unavailable actor) or by request. In a television series, a protagonist will usually be an actor regularly reprising the role for each episode, unless external circumstances leave him or her unavailable.
The script is also broken down into several scenes, and translated to action through the use of storyboards (rough sketches of each scene with captions indicating actions not visible in the former), and all the locations, props, cast members, costumes, special effects and visual effects are identified. Sets and props are constructed, costumes are designed, and a production schedule is set forth. Scripts are given to the cast members for them to read and memorise their lines, and a special meeting is organised to let the cast read through the script as a group; staff and publicists are also present to serve witness.
In the case of Doctor Who, most television stories had completed pre-production and exist in at least audio form. However, a good number of planned stories were dropped during pre-production for numerous reasons, most notably the original Season 23 of the show, which was completely shelved in favour of the Trial of a Time Lord story arc, and The Final Game, which was dropped following the sudden death of Roger Delgado.