Under no circumstances...

circumstances--conditions motivating or surrounding persons, places, things, and events in story; the status of behavior related to a character, that character's agenda and/or vulnerability, and the likelihood of a predictable behavior manifest in the actions or failure to act in a particular character.

Mysterious or ambiguous circumstances surrounding a character impart an atmosphere of menace and uncertainty. The suspense grows as the circumstances become volatile, leading to the growing apprehension that such a mysterious or ambiguous character will be driven to the point of combustion, whereupon he or she will reveal a true self of incredible awfulness.

Familiar, even predictable circumstances advertise a growing sense that a surprise explosion is building, some unanticipated demonstration that will ruin the stasis, nudge it unceremoniously over the edge into chaos and discomfort.

Thus the mysterious and the familiar combine to frame events and the persons who participate in them, raising suspicions wherever possible. These suspicions translate to tension which, lacking actual suspense, is an excellent glue for holding story together. Readers wonder what new circumstances will bring to the equation of characters, bent on pursuing an agenda, meeting frustration and reversal.

All a protagonist or antagonist needs in order to set a story in motion is to hear the repetition of the familiar mantra "Under no circumstances..."