King and I agree that characters are the key element of a successful story. "The best stories always end up being about the people rather than the event, which is to say character-driven," he says. How many times have you read a book with an awesome premise, only to be disappointed with the character living it?
King's advice for creating successful characters is essentially the same axiom every writer has heard ad nauseum: "Write what you know." But King elaborates-- just because you're a plumber doesn't mean you need to write about just pipes and toilets.
"Write what you like, then imbue it with life and make it unique by blending in your own personal knowledge of life, friendships, relationships, sex and work. Especially work."
Despite several criticisms, he uses John Grisham as an example. A former lawyer, Grisham has used his personal experience and made a world
"impossible not to believe. Grisham has been there, spied out the land, and the enemy positions, and brought back a full report. He told the truth of what he knew, and for that if nothing else, he deserves every buck The Firm made. ... Grisham's make-believe tale is solidly based in a reality he knows, has personally experienced, and which he wrote about with total (almost naive) honesty. ... John Grisham, of course, knows lawyers. What you know makes you unique in some other way. Be brave. Map the enemy's positions, come back, tell us all you know."
I don't think King means you should only write things you know-- especially considering he ends the section by saying "And remember that plumbers in space is not such a bad setup for a story." But something in your characters has to stem from personal, visceral, genuine experience before you can combine it with your brilliant plot idea.