たちまち

たちまち is a strong adverb that describes a rapid, almost sudden change or reaction. It emphasizes that once something happens, the situation changes or a reaction occurs immediately, without noticeable transition. It is often used with verbs like 変わる, なる, 落ちる, 騒然とする, 沈む, 回復する, etc., and can describe both external events (weather, circumstances) and internal states (emotions, opinions). It conveys a sense of immediacy and dramatic shift.