4. Principal Photography
This where the actual Cameras roll.
This will usually always be the most difficult stage of the production to get right, as it will involve all the cast and crew and other departments putting their different plans into action, working in unison to achieve a common goal.
It is also usually the most expensive stage of the production (unless its a CGI heavy project) as it will involve the directors fees,cast and crew salaries, not to mention the cost of certain shots involving locations, props or on set special effects such as a shot you can only afford to do one take.
Everything you do in previous stages up to this point is to ensure this stage of the production runs as smoothly as possible.
The production must keep a full set of records and strive to ensure the production stays on time and on budget.