There are times when I plan a shoot to the smallest detail. I will have the subject in mind, I will check my gear, set up lighting, reconoiter the site, and in everyway possible be prepared to shoot exactly what I have in mind.
On other days, I simply walk around with my camera hanging off of my shoulder and let the shot come to me. Both situations can be rewarding. In the first, I enjoy seeing the positive results of my hard work. In the second, I am wrapped up in the moment of unplanned inspiration. In each situation, I know that I have a great shot in the camera the moment I press the shutter button.
To me being a great photographer has to be a mix of hard work and inspiration. When I become one, I will let you know if I am right.