Sketching already takes me one step into the painting, it prompts my internal dialog: "what kind of brushstroke will I use for this stretch of hair? which "strands" should I make dark and which should I make gray? how light should I make his long gray beard? how far can I obliterate detail in the face area and still leave enough of a sense of expression? is it important to leave some sense of the man's age, or is that irrelevant? how can I portray his gentleness and peace and avoid making his face hidden in shadow scary? how much do I need to describe his t-shirt? how much overlap to create between his hand and his head? which edges should be lost, and soft, and which should be hard, and found?" ...and I'm not even thinking color yet!
Those questions were set in motion and now will live in my mind, much like characters do in a for a writer during the writing of a novel, and I know the answers will oscillate back and forth until I finally pin them down in the painting itself.