These words refer to actions that are taken on the spur of the moment or without forethought. Spontaneous includes both these ideas and may often include, as well, suggestions of naturalness, frankness and good humour: the child’s spontaneous answers to all out questions. Sometimes spontaneous is deliberately contrasted with such words as routine or conformist: the spontaneous exuberance of teenagers when they feel free to be themselves. In another use, the word is sometimes restricted in reference to voluntary rather than coerced action or to action that comes about by general agreement arising out of immediate circumstances: a committee; a spontaneous protest demonstration that began with a few disgruntled students and ended in a march on one of the administration buildings. Impulsive is far less positive in its implications as compared to spontaneous. It may, in fact, suggest someone governed by or at the consequently, lacks the overtone of good humour present in spontaneous and can apply to ugly or disruptive actions as well as to pleasant ones: an impulsive generosity that alternates fitfully with equally impulsive temper tantrums. In some cases, the word can be used to describe actions occurring on the spur of the moment: supermarkets that display their wares to encourage customers to make impulsive purchases of items not on their shopping lists.

Unplanned is the most neutral of these words in stressing only the lack of forethought and carrying no emotional overtones about the quality of the action: an unplanned stop-over in Djakarta because of questions in depth. Unpremeditated is the most technical of these words; in legal terminology, it refers to an impulsive crime committed without forethought and therefore unplanned . This would be less serious than the same crime devised in advance. Except in a legal context, it would sound stiff as a substitute for unplanned or impulsive , unless a comic touch were intended: a party that turned out to be an unpremeditated disaster.

The remaining words pertain mostly to a context of public speaking or musical and theatrical performance. Extemporaneous refers specifically to a speech delivered on the spur of the moment without notes, but especially without a written version of the speech to be given: ministers long used to working up overnight an extemporaneous Sunday sermon; the question-and-answer period of the television debate that by its very nature had to be extemporaneous . Impromptu pertains, most specifically, to a kind of musical performance in which the music played is invented as the performer goes along: an overblown piano fantasy that was surely impromptu . The word is often used also of speeches delivered on short notice: impromptu speeches tacked on the each seconding motion. In this area, the word can be distinguished from extemporaneous in that the latter may be a matter of choice, even when the speech has been set long in advance, whereas impromptu suggests being called on to speak when one is not expecting it and is therefore of necessity unprepared.

• The teacher gave the same extemporaneous reprimand, word for word, to every student caught cheating; nor did he flicker an eyelid as he listened to their impromptu replies.

Both unrehearsed and improvised can be used in either a musical or a theatrical context. Unrehearsed , here, may suggest that a set piece is to be performed, but that the players have not previously played it through. In this context, improvised would suggest a basic structure within which the players have considerable opportunities for spontaneous invention. Although improvised is generally used in a neutrally descriptive way, unrehearsed can sometimes suggest a negative judgement: Jazz is necessarily improvised , but it is ridiculous to think of it as unrehearsed . In a more general context, unrehearsed approaches spontaneous in its implications, stressing a voluntary and unplanned telling or acting out: He asked bluntly if the witness’s evidence was freely given and unrehearsed . Improvised can suggest a rough-and-ready substitute for something lacking or the making of decisions as one goes along: an improvised tent-pole made from a stripped branch; an improvised tour, taking them from place to place as the spirit moved them.