Though I talk a lot about how professional writers produce copy on schedule, not upon inspiration, there are times when the white heat of inspiration strikes all authors. They must get the idea or the phrase or the words down immediately lest they lose them forever.
Long ago, the great American writer Henry David Thoreau wrote: "Write while the heat is in you. The writer who postpones the recording of his thoughts uses an iron which has cooled to burn a hole with."
Get a journal, a voice recorder, or something that will enable you to get the words from your brain to the page. There are moments when every writer has flashes of genius. Keep a notepad on your nightstand. I don't know how many times I've awakened from a dream with a sentence or an idea that I wish to remember.
If you're driving and this happens, phone your home and leave a message on your answering system that records your brilliant idea. Or set up an account with a voice-to-text service. Many are free, or, like Dial2Do, charge only a small amount each month.
Find a way to capture moments of pure inspiration because they're a gift.