The problem with working on assignment is that it takes the curiosity out of the process because you're really conscious about what you ought to be doing. Most of the pictures in my books are pictures taken between assignments, or after assignments, or occasionally even during assignments, but not for assignments. When I work for myself, I do whatever I please. Generally, those are the better pictures. When you're working on your own, you're just looking for pictures.
Luck is very important, and some people are luckier than others.
You see a situation, you wait around, and maybe something happens. I very often bark at dogs to get their attention or get them to react. Once I barked at a dog and the owner turned around and kicked it. She thought it was the dog barking. I sometimes use my own dogs in my pictures.