One of the best techniques for capturing unexpected events is to preset your camera. I preset mine all the time. I also code my cameras for every shoot. That helps with the tracking of the images when they get all mixed up on sites and on the computer. It also helps with proving ownership of images.
The image for this article was shot going over a drawbridge right at sunset. The image is somewhat a surprise to me as the camera was not set for this type of shot. It was a true pick up the D700 and shoot. I do use back button focusing so I did not lose any time with the lens refocusing when I pushed the shutter release. The fact the image was shot in front of me helped. If I had been traveling across the trees the whole image would have been very blurred.
Remember that subjects coming at you or going away from you are much easier to keep in focus or to keep them looking better than ones crossing across your vision. Those will require a much higher shutter speed to keep from being really blurred.