On the courses I run and talks I give people are often surprised that you need to be ready to get that shot at all times. As a street photographer you never know when that great shot will come round the next corner. People I have found fail to be ever read and also fail to read the landscape/surroundings as they move through an area. These for me are key aspect and this short video is a nice explanation of the how and the why.
Are you ready to photograph?