Think of act as simply doing, and acting as the doing that is going on right now, inside a role that is not permanently yours. On a stage the role belongs to a character: for two hours you speak and move as someone else, and then the lights go down and you stop. Take that picture off the stage and the same shape turns up twice. Someone holds a job that is not theirs to keep, and hands it back when the usual person returns. Someone shows a feeling that is not theirs either, and drops it when nobody is watching.
Acting is act + -ing. Act runs back through Middle English acte and Old French to Latin agō, 'to do, to drive'. The doing is the old part of the word; the -ing is what turns it into the doing that is under way.
「acting」是 act 加上 -ing。act 經由中古英文 acte、古法文，一路回到拉丁文的 agō，意思是「做、推動」。「做」是這個字最老的核心，-ing 則把它變成「正在做的那件事」。
Wiktionary etymology tree supplied in this packet (source_ref kaikki:3f6f69a190036ae4:434067384): PIE *h₂eǵ- > Latin agō > Old French > Middle English acte > English act + -ing.
From the picture
暫時頂著別人的職位
一樣是「扮演一個角色」，只是角色換成了職稱。原本的人不在，你先站上去，他回來你就還給他。
adjective · B2
Acting manager, acting director, acting head teacher and so on: the person doing that important job for a short time, because the usual person is away, or because nobody has been chosen for the job yet.
代理（職務）
noun · B1
The work or the art of playing a character in a play, a film, or a television show.
演戲；演技
noun · B2
Behaviour that someone is putting on: they do not really feel what they seem to feel, they are only pretending.
裝出來的；演的
以下是外部詞典記錄的用法，原文照列，我們還沒有寫成教學內容。上面的說明才是我們自己寫的；有些用法可能上面已經講過了。
actingadj/ˈæk.tɪŋ/
Temporarily assuming the duties or authority of another person when they are unable to do their job.
actingnoun/ˈæk.tɪŋ/
archaicAn action or deed.
lawSomething done by a party—so called to avoid confusion with the legal senses of deed and action.
Pretending.
broadcastingdramadramaturgyThe occupation of an actor.
這個拼法也被記錄為下列字的變化形：
資料來源：英文維基詞典 English Wiktionary（CC BY-SA 4.0）。 原文照列，未經改寫。