Idiom(s) section. be/act your age to behave in a way that is suitable for somebody of your age and not as though you were much younger act/play the fool to behave in a stupid way in order to make people laugh, especially in a way that may also annoy them: Quit playing the fool and get some work done! behave/act as if you own the place | think you own your place (disapproving) to behave in a very confident way that annoys other people, for example by telling them what to do.
Phrasal verb(s) section. act for/on behalf of somebody to be employed to deal with somebody's affairs for them, for example by representing them in court act on/upon something to take action as a result of advice, information, et cetera: Acting on information from a member of the public, the police raided the club; Why didn't you act on her suggestion? act something <-> out 1 to perform a ceremony or show how something happened, as if performing a play: The ritual of the party conference is acted out in the same way every year; The children started to act out the whole incident. 2 to act a part in a real situation: She acted out the role of the wronged lover. act up (informal) 1 to behave badly: The kids started acting up. 2 to not work as it should: How long has your ankle been acting up?