Pre-visualisation — often reduced as simply pre-vis — is a stage in the pre-production process between storyboarding and principal photography. The skill is not known to have been a part of the 1963 version of Doctor Who, but it has been used with uncertain frequency on the BBC Wales version of the programme.
Though Doctor Who titles have consistently spelled it "pre-visualisation", it's comparatively unusual to see it as a hyphenated word. It's typically spelled in American-led circles as "previsualization" or in British-influenced projects as "previsualisation". Thus, its shortened form is often styled "previs" — or sometimes "previz".
The skill involves translating static storyboards into a full-motion approximation of the final scene. Though pre-vis can technically be accomplished without CGI — as, for example, with cheap, hand-held video cameras — computers have provided a low-cost, more flexible way for a pre-vis artist to contribute to the pre-production pipeline.
Very rough models of the sets, locations, actors and other in-scene objects are input into a computer programme which has the ability to precisely emulate physically possible camera movements, speeds, and sometimes even cinematographic details such as lens aperture and lighting physics. Following the storyboard, the pre-vis artist then brings the scene to life, thereby offering the director a much more detailed guide on how to film the scene. The process can reveal technical flaws in the storyboarding, which prevents the director from wasting time on approaches that won't work. The process also offers the director the opportunity to try different shots in the computer before he commits to them on camera.