a.k.a. "shooting," this is the phase of acquiring all the footage the project requires. The footage can be interviews or what's known as "b-roll," which consists of the visual stories that support what's being spoken in the narration and talking head interviews.
The costs are affected by the number of crew personnel required (from a one-man-band who is simultaneously shooting, recording sound, and directing, to a large crew of people performing specialized functions like lighting, management of props, makeup, etc.) and by the equipment requirements.
One factor in designing this stage of the project is the convention of shooting full days rather than partial days or by the hour. The rationale is that for many crew members a commitment of part of a day on one job means they are not available for a possible full day on another job. Most of the time it's best to design the budget using full days of shooting, and therefore to schedule as much shooting as possible (within reason!) in that time frame.