You may see something really groovy happening and happen to have the camera nearby, or you may have an idea--a concept--already in mind. Maybe it's a combination of both.
I went down to the canal with no concept, no real fleshed out idea. Just needed to go out and shoot. I spent about two hours walking around, looking. If something interesting happened--like a heron taking off from a perch--I quickly tried to capture it. (I did! Looks cool, too. Will see it here at a later date.) A couple of bicyclists or joggers headed my way, got them too. Then there was the shot you see (sort of) below. I found myself standing there watching the water rush by, listening to the birds, the wind in the trees... taking it all in.
In one direction I noticed an interesting possibility for a shot. Bringing the camera to my eye, I began looking for just the right composition. Moved a little to the left, then right, then up a little, walked forward.... Without tripping the shutter I kept moving around until it felt just right. When it finally did (sometimes it never does!) I took a shot and examined it on the LCD screen. Looked good there, too... but it needed the human element. I put the camera on the tripod, recomposed the shot and pre-focused to a spot down the towpath a ways. Then I waited. It was still a little early so foot traffic was still light. I decided to set the timer and at least get a self-portrait. Ultimately, people did ride, walk, run by in each direction. Got a few images to choose from and will probably go back for more... maybe bring a model just in case!
In the mean time, here's a double self-portrait.