In any particular drawing or painting there is an overriding idea you are working with. For example in the drawing below I was very interested in the dramatic lighting of the character. The light was very stark and I had to choose a side to join: I can only make a limited range of values with a pencil. Would I use my available tones for the Dark side or the Light side? You can see that I chose the dark side as I used almost all my tonal values describing the shadows. I left the light side of the range almost completely as the white of the paper. This focus on the shadow side increases the drama of the piece and complements the subject's pensive pose.
Other times, like this concept sketch for the Gilded Butterfly painting, I'm working out the overall shape and flow of the image I want to create.
20" x 16"
Oil on Panel
I often use a sketch as a way for my mind to work out visual ideas. Sometimes it is helpful to have a visual note of my intentions to look at as I work.
Not all drawings are intended to be complete finished works of art. In fact, some just don't work out at all. Although I will say, often, because of their strong focus on one idea or element, the process sketches themselves can be beautiful in their own right.
So don't be afraid to play around and risk some ugly drawings, oftentimes it is the happy accidents in sketching that have given me the inspirations for my favorite paintings.