Click.

In the author’s mind there bubbles up every now and then the material for a story. For me it invariably begins with mental pictures. This ferment leads to nothing unless it is accompanied with the longing for a form: verse or prose, short story, novel, play or what not. When these two things click you have the author’s impulse complete. It is now a thing inside him pawing to get out. He longs to see this bubbling stuff pouring into that form as the housewife longs to see the new jam pouring into the clean jam jar. This nags him all day long and gets in the way of his work and his sleep and his meals. It’s like being in love.

C.S. Lewis, from Of Other Worlds