When filming, every single element has to be planned for and scrutinized. From the lights at a location to the exact set-up of each prop and furnishing, it is important that everything was designed with a specific purpose in mind. Before this can be done for a given location it is important the locations undergo an intensive technical scout so that each department can focus in on how to work with that area.
The technical scout is often referred to as a reconnaissance mission because the lead in each department is going to that location to see the space and any problems that they might have with it. This scouting party needs to consist of a large number of people, from the Unit Production Manager, to the Assistant Directors, to the Art Director. Often times the producer will not attend, but on smaller films it really is advisable. Once there the director will lay out the scene for all the crew members so they will know exactly what is going to occur and what they will have to do. This is a complicated process that requires each department to figure out how things must be done. For example, the sound department will have to figure out how to set up microphones to get the correct kind of pick-up while blocking out any outside competing sound. All things must then be accounted for, from the coordination of practicals like food and transportation, to how any alterations must be made for aesthetic and technical purposes. It is likely that one day will be spent to analyze and then confer with the entire department, and then the second day will be the preparation. This dual day process is quick but necessary when there are a number of locations that must be dealt with. If you only have a couple of locations more time can be spent. Either way this must be comprehensive as it ends up being one of the most important preparation periods of pre-production.
This is an important creative process as the actual vision of the director is being applied both for the screen and the realistic expectations of a production. Without a streamlined process the production process could falter and fail to meet the set expectations. The Unit Production Manager and first Assistant Director often coordinate all of the departments during this process, though the director still has final say. This cannot be done too early, but should go at least a couple weeks before primary photography begins.