I speak for spontaneous painting.
Sometimes I simply see something, and think: “That would make a great painting” or I just make it up as I go along, often working from memory. That is what I describe as “spontaneous painting.”
I also think of the old cartoon of the painter, brushes in hand, shouting to the model: “Hold That Pose!”
Spontaneous painting (or drawing) presents the opportunity to shut off the mind and the invasive planning that we often do. The monkey brain leaping from branch to branch, or, in the opposite, studying and planning every movement.
To be spontaneous is to accept the moment, and accept the possibility of both success and failure. Look, and Leap. The end product, be it a painting or drawing, must then be accepted for what it is – an exploration. No one need to get upset; there is no need for apology. Certainly, fussiness is a vice in this situation. The temptation is to overthink it, then overwork it.
What is left on the canvas or the paper is a record of what happened in the moment, in a way a bit of a self-portrait, more honest than any photograph. Nobody posed – it just happened.
If one can accept the risk, one learns do much. More importantly, one has lived through a moment of direct experience and direct action, both doing and not-doing. There is always what you leave in and what you leave out. Detail is precious; only go so far as it appropriate. Only include what is essential, and never allow yourself to fall into some concept. That is like adding too much salt to the soup.
Rather, allow what you are seeing to guide you.
After all, the moment will be gone soon enough. The light will change. The goal of spontaneous sketching or painting is to find that time of the meeting of clarity and harmony: what is, what should be, and what should never be. As the Buddhists say, “First thought, best thought.”