The answer is; I don't have a clue. Almost never do.
Gag cartoon, editorial cartoon, a novel, the first chapter. Even an e-mail. Or a text message.
No joke and no exaggeration. When I sit down to draw or write I'll have a vague feeling or notion or general sense of a situation, or an image of someone or something; or a memory will appear and launch off into never-never land.
My imagination—and frankly, I believe your imagination, too—is its own engine. It operates on a totally different frequency. It's not in the same place as a chat with the checkout person at the supermarket or my neighbor, or a thank you note. Those are down here: in the tangible, factual, living, breathing world: of facts, information, reports, numbers. If we could create a chart, a graph, a visual representation of relative locations of the different products of our mind; I might use a map of the universe, earth, moon, planets in concentric rings.
A déjà vu—my gift from migraine; don't ask—would be in the outer ring; space beyond our system; the unknown; may be connected in some way but not a part of the universe as we know it. Yes, it is a product of my brain, but to me, it's a compartment that's totally unconnected to my self; mimic of a stroke—which I'd put in the same place—incomprehensible, out of my control.
Nearer, on the map, would be a dream; which does have a connection to my life; my mind; experience; wishes; fears. Needing interpretation, but certainly engaged in my life, my plans, my concerns. The works.
Creative imagination would be adjacent to a dream. But while a dream might relate to a wish or a concern—an experience wearing a disguise, needing translation—creative imagination is the raw material: free-form, floating, but a source, a reservoir, a stimulus for producing a tangible product. Ready for use. In the world of science, not infrequently, it's a what-if that begins the process. Taking a challenge and running with it: a societally useful goal, a self-driving car, a visit to the moon. Achieving that goal, using the full arsenal of every bit of material within reach: understanding from basic principles to ethereal concepts, using available elements in a new way, or inventing the new concepts. The brain as the "How" after the "What if?"
The creative world has one foot here and one foot who knows where. Setting off on an adventure that, in a way, is as surprising to the creator—me, in this case—as to the observer; who asks "Where did that idea come from?"
Answer?
Let me show you this chart.
You see, here's a dream out here…