From composer Igor Stravinsky: "The more constraints one imposes, the more one frees one’s self."
It’s counter-intuitive, but true.
Consider three co-executioners, none of whom will ever know whose syringe contained the poison. But one grows sure it was his, and he makes peace with it. Another grows sure it wasn't his, and makes peace with it. The third remains unsure, and suffers for it.
Consider a writer. "What shall I write about today?" launches me into an untethered, abstract infinity. It's better to begin at ground level with constraints -- details -- and let their energies develop into a story.
Consider story characters. When mine need to move -- but instead just stare at me, frozen in indecision -- I try the “bracketing” technique from James V. Smith’s Fiction Writer's Brainstormer. It’s a method of generating options between the extremes of possibility. Take a story question, any question (“Where is she going in the car?” “What does he do when his boss hangs up on him?”), and first give the automatic answer and then the over-the-top answer. Between those extremes lie lots of interesting alternative answers: the commonplace, the odd, the opposite, the adolescent, the inventive, the romantic, the magical, the obscene, the math-related, the biblical, the amusing, the poetic, etc., etc., etc.
I remember using the technique to brainstorm occupations for a story’s accountant-type secondary character. Through bracketing, the probability emerged that he was a circus acrobat -- totally ridiculous in my serious story. Until ... constrained to resolve that ridiculousness by weaving it into the plot ... I stumbled upon the story’s central secret.