I struggle to imagine a fixed image or object on demand, but when falling asleep or in certain situations its like something gets unplugged and then everything becomes way more vivid and detailed to the point of becoming a flood. I've often wondered if artists can naturally get into that state much more easily.
If you do some contour drawing exercises, you'll notice right away how much you can fill in just by getting a decent silhouette: a lot of systems of figure drawing simply go into more detail with that and aim to codify rules for what lines are necessary.