I have trouble with sketching. I tend to view it as an end in itself, not something on the way to another painting. So when I am en plein air, I sketch in my notebook before I paint, to understand the composition. But when not en plein air I tend to go straight into a painting, using a photograph or my magic book of ideas, which are thumbnails of shapes, colour ideas, titles even. I may even start a painting entirely based on my idea for a title, or basic shapes/colour conjunctions that came to me in the shower. The latter are the most imperative.They may lead to me exploring compositions to fit the original idea or to abstraction. Usually the resulting paintings are energy filled as I must get them down onto the support as soon as possible.
Instinctive, you're right. Like the cave painters of the Palaeolithic in France. An elephant is painted onto a rock shape which has reminded the hunter of the animal. Gaughin would go along with that, I feel.