The key ingredients are unlimited no-cost film, unlimited and expedited no-cost lab processing, a fast acting camera, a paying audience hungry for "candid" pictures, and most importantly the devouring obsession to keep doing it and doing it. The decisive moment, if there is one, doesn't happen at the camera-work stage but rather when the yellow wax-pencil marks the key frame on the contact sheet.
A classic example of the genre is, of course, Henri Cartier-Bresson whose savage compulsion to expose film yielded a rare (for the times) bunch of pictures that delighted the public and the editors of the first generation of mass circulation picture magazines. Now digital picture-making can generate avalanches of images attended by no thought, no effort, and no cost and poor Henri would be swamped by today's overwhelming visual clutter. And the mass circulation picture magazines that showcased Henri's obsession are long gone too. Sure, there are brillant digital picture-makers working today but you won't see them in print. They are on your computer monitor if you can find them among the millions clamouring for your attention.