You know how when a horse walks into a bar, the bartender asks: "Why the long face?" When an idea walks in, an author never asks why. You sit that idea down, pour it a drink, and listen to everything that idea has to tell you. An author should listen more than she writes. You listen to your ideas, and you entertain your ideas. Maybe you get them another drink to get them tell you more than they'd intended. Give them your rapt attention.
Never say, "Well, that's not the sort of thing I normally write." Don't get stuck within your self-imposed borders. Don't get stuck within borders that others impose upon you. In writing, as in many other careers, once you get good at something, the temptation to stay there in that place is overwhelming. That's the level you know. That's the level that pays the bills. That's the level where publishers, or whatever industry you work in, make money off you. The people who make money off you will work to keep you in that place. The requirement becomes to do what you do over and over. Experimenting takes time away from producing. Experiments can fail. Nobody wants the downtime. Nobody wants the dead-end streets that eat up energy. Yet look at nature. In the spring when I go outside to check on our trees, I look for new shoots. If I see no buds, I know that tree had died during winter. Only where there is new growth does life exist.
Have a fleeting, runaway idea? Run after it. Chase it even to the ends of the world. Expand your world. Expand yourself. Our most important ability in this life is to grow.