In these posts, I often write about being properly prepared before you start shooting pictures. This is a good example of why that is the case.
If you are looking to capture a candid moment, it only comes once. That means that you have to create the situation where your camera can capture what happens as events unfold. You need to look at how fast of a shutter speed you need to freeze movement (or show it). Shutter speeds of 500th of second or more will probably freeze all but the fastest action. Slower shutter speeds like 1/30 would show blur.
You want to think about the depth of field, and how much you want to keep in focus. Shallow depth of field, maybe f4, means the area covered by the focusing point in the camera will be sharp, but objects closer or further away will go fuzzy. Setting the camera to maintain a large amount of depth of field, like f16, will keep more things in focus front to back. This can be an advantage when people are moving around a lot because there is more fudge factor for the camera focusing system.
And you want to think about angle of view and how that affects how you feel the action itself. A wide angle lens will bring you into the action more, whereas a telephoto lens will flatten things out and make you feel more like an observer than a participant.
In this case, I shot with a 35mm lens, at 1/400 at f14. I intentionally chose to shoot from close in, and below the boy so I could see the emotion in his face and feel more a part of the scene. I chose to shoot at 1/400 because I knew that when the snowball was smashed, some movement would still show in a blur. And I shot at f14 so that I could keep both people in focus, even if they moved around a little bit.
But remember, I decided all these things BEFORE we started shooting. With the camera set up properly, I could simply focus on the activity, and make the composition work as best I could. I highly recommend this approach to the creative process, and in shooting action, the benefits are clear.