I don't finish drawings, I exhaust the possibilities in each rendering. I revise and rework and more than likely destroy drawings. I draw until the figure and the environment have a visual weight (a plasticity) and can carry a narrative. I draw until I find a hook that sustains my viewing for more than a couple of days. If that hook doesn't last, then I go back to revising. Whatever began the drawing – the studies, the images from my boxes - is started again when I pick up my piece of charcoal. Because the mark is not the idea, I have to battle what making that mark means. Does it define the outside or the inside of the object? Is it defining a texture, a contour, or a tone?
Since most of us have mark making down, we think it's automatic. At its fundamental level, a drawing is a progressively complex listing of strokes. Nothing more. To assume that you can express anything before controlling and manipulating the material is ridiculous.
That's not to say that you don't have a start. I have plenty of starts, plenty of ideas, but once you are on the paper it's a whole different game.