With documentary filming one tries to keep an even exposure throughout a scene. You never know what is going to happen. When you've been doing it long enough you develop  a feel for exposure variation by the amount of light coming through the eyepiece (You always see what the lens is seeing because the mirror shutter spins at the back of the lens, allowing light to the emulsion when it is open, and reflecting light to the eyepiece when it is closed) - so you could make manual adjustments to the F-stop.

The more variable the light - say having to move and shoot through differently lit spaces - the more one used wider lenses with greater depth of focus. Changing stop and focus at the same time was tricky ;-)

You can't be me, I'm taken