We all have to find our own way of working. When working with clients ideas are usually hashed out before hand. I prefer to work with a general feel or tone of what I need to produce and then leave me with the freedom to create it. I will usually shoot what was in that context and shoot my own interpretation. Sometimes the client goes with my version, sometimes not but what seems to have happened in my case is more often than not I am given a good deal of freedom to create and get hired for my vision.
For my personal work I usually work in the context of bodies of work and will have a basic feel for what I want but again I will just work allowing it to take me where it leads me. Sometime what you come back with is not at all what you set out for but thats OK. Sometimes there is a really good image that doesn't fit with anything else in the greater body of work. Thats OK. Ralph Gibson calls them "points of departure" which can be the birth of a new project or new direction.
Here are a the way many of the greats worked.
"I start with no preconceived idea - discovery excites me to focus - then rediscovery through the lens"-Edward Weston
"One does not think during creative work, any more than one thinks when driving a car. But one has a background of years - learning, unlearning, success, failure, dreaming, thinking, experience, all this - then the moment of creation, the focusing of all into the moment." -Edward Weston
"'Manufactured' or staged photography does not concern me. And if I make a judgment, it can only be on a psychological or sociological level. There are those who take photographs arranged beforehand and those who go out to discover the image and seize it. For me, the camera is a sketch book, an instrument of intuition and spontaneity, the master of the instant which - in visual terms - questions and decides simultaneously. In order to "give a meaning" to the world, one has to feel oneself involved in what he frames through the viewfinder. This attitude requires concentration, a discipline of mind, sensitivity, and a sense of geometry. It is by great economy of means that one arrives at simplicity of expression. One must always take photos with the greatest respect for the subject and for oneself." - Henri Cartier-Bresson
"The state of mind of a photographer while creating is a blank...For those who would equate "blank" with a kind of static emptiness, I must explain that this is a special kind of blank. It is a very active state of mind really, a very receptive state of mind, ready at an instant to grasp an image, yet with no image pre-formed in it at any time. We should note that the lack of a pre-formed pattern or preconceived idea of how anything ought to look is essential to this blank condition. Such a state of mind is not unlike a sheet of film itself - seemingly inert, yet so sensitive that a fraction of a second's exposure conceives a life in it. (Not just life, but "a" life)." - Minor White
There are those that work just the opposite. Whats important is finding the way the works for you.