Act, behave, work, operate, function and react all are comparable when used with reference to the way in which a person or thing does what is expected or responds to external influences or circumstances.
Act is not only the most general word of this group but also the most general of all English intransitive verbs except those (as be, exist, belong) which assert being, a state of being, or relation.
Act is therefore used largely in interrogative sentences when knowledge of the specific nature of the action is sought or in declarative sentences with a qualifying adverb, adverbial element, or adjective complement.
- how did the child act when you called him?
- he acted as if he were about to cry
- he acted frightened
- how should this powder act when mixed with water?
- this medicine acts as a poison to some persons
Behave is widely applied chiefly to persons and their conduct with reference to a standard of what is right or proper or decorous.
- one must keep one’s contracts, and behave as persons of honor and breeding should behave
- However, in or parallel to technical use behave often approaches act in generality.
- study how steel behaves under stress
- how the thyroid gland behaves during emotional excitement
- two men may behave like a crowd . . . when their emotions are engaged
Work, operate and function agree in meaning to act in the way that is natural or intended.
- the Swiss clock had long since ceased to work
- but she had not thought. Her brain would not operate
- they have functioned as observers rather than participants
— J. M. Browny
In distinction from one another work may, especially when qualified, suggest success or effectiveness.
- the fact that a theory has actually worked is a better recommendation for its soundness than any amount of ingenious dialectic
Operate stresses efficient activity rather than achievement except when followed by on or upon.
- the revolutionary spirit, ceasing to operate in politics
Function implies activity with reference to the accomplishment of the end or office for which a thing exists or is designed.
- consciousness ceases altogether at death, when the brain no longer functions
- rules of the game which must be observed, if society is to function at all
React, a word of rapidly shifting implications, is often used as though it were a close synonym of the preceding words, especially of act or behave.
- at this threat the civil service reacted in the way which is always open to any civil service, under any regime
—C. P. Fitzgeraldy
In discriminating use it always suggests recoil or rebound; often more narrowly, but still consistently, it implies reciprocal or counteractive influence or a reverse effect.
- home and the school react (act reciprocally) on each other
- whilst most people’s minds succumb to inculcation and environment, a few react vigorously: honest and decent people coming from thievish slums, and skeptics and realists from country parsonages
As a result of use in chemistry and psychology, react now often implies a favorable or desired response.
- children react (respond favorably) to kind treatment