Henri Cartier-Bresson was asked, “Have you ever really been able to define for yourself when it is that you press the shutter?”
And he answered
It’s a question of concentration. Concentrate, think, watch, look and, ah, like this, you are ready. But you never know the culminative point of something. So you’re shooting. You say, “Yes. Yes. Maybe. Yes.” But you shouldn’t overshoot. It’s like overeating, overdrinking. You have to eat, you have to drink. But over is too much. Because by the time you press, you arm the shutter once more, and maybe the picture was in between.
Very often, you don’t have to see a photographer’s work. Just by watching him in the street, you can see what kind of photographer he is. Discreet, tiptoes, fast or machine gun. Well, you don’t shoot partridges with a machine gun. You choose one partridge, then the other partridge. Maybe the others are gone by then. But I see people wrrrr, like this with a motor. It’s incredible, because they always shoot in the wrong moment.
From a 1971 interview with the photographer by Sheila Turner-Seed. Published in the New York Times, June 21, 2013
The background of the interview is interesting and included in the article.
Waiting For Charlemagne 