acting
Word family
Synonym discussion
Sense: adjective/1
All four describe someone or something that fills a role for a limited time, but they differ in formality and scope. 'Acting' is the standard label for an officially appointed substitute in a senior post (acting president, acting director); it always sits directly in front of the job title. 'Interim' is very close and often interchangeable, but feels slightly more business-formal and is common in press releases — 'interim CEO'. 'Temporary' is the broadest word: it covers any short-term arrangement (a temporary worker, a temporary fix), not specifically a leadership stand-in. 'Stand-in' is informal and suggests a one-off cover, like a stand-in teacher for one afternoon, rather than a multi-month appointment. Compare: 'the acting mayor signed the order' (formal title) vs 'a stand-in teacher took the class' (casual cover).
Common collocations
Etymology
Both senses come from the verb 'act', borrowed into English in the late 1300s from Latin 'actus' (a doing, a performance), the past participle of 'agere' (to do, to drive). The noun sense ('performing on stage') developed in the 1500s as English theatre grew, when 'acting' came to name what an actor does. The adjective sense ('temporarily holding a role') appeared later, in the 1700s, drawing on the older meaning of 'act' as 'carrying out the duties of a position' — so an acting captain was someone doing the captain's work for the time being. The two senses thus share one root verb but split early into very different uses.