Artists have often used drawings or under-paintings "beneath" their paintings, sometimes done in charcoal, sometimes painted in loose washes. This helps to get the idea down in visual terms, as a starting point. Then the painting itself takes off from there.
Last week, I did a black and white paper collage, as a way to test composition. But then I thought, "Hey, what if I just use this as an underpainting and paint right on top of the collage?" So I glued the collage down to a piece of foam core board and coated the whole thing with matte medium, to seal the collage.
I really had no idea, at that point, where this painting was going, but I was eager to find out!