Precipitate, headlong, abrupt, impetuous, hasty, sudden as applied to persons or their acts or behavior denote characterized by excessive haste and unexpectedness.
Precipitate especially stresses lack of due deliberation; sometimes it suggests prematureness and is therefore especially applicable to decisions or to actions based on decisions.
Headlong throws the emphasis on rashness and lack of forethought; it is used to describe not only persons and their acts but the qualities exhibited by such persons or in such acts.
Abrupt when applied to actions suggests complete lack of warning or, sometimes, unceremoniousness and when applied to manners or words, it usually implies curtness.
Impetuous implies violence or vehemence; as applied to persons, it often also suggests impulsiveness or, at times, extreme impatience.
Hasty stresses quickness of response and often suggests thoughtlessness and hot temper rather than impulsiveness.
Sudden is distinguishable from sudden meaning unexpected only by its added implications of extreme hastiness or impetuosity.