Picture one office with a very long list of actors. A film calls and says: send us a bank manager, two nurses and a taxi driver. The office does not send interesting people; it sends the ones who look the part, because that is what the film asked for. Everything this expression does comes from that office and the kind of person it sends.
「可以想成一間辦公室，手上有一份很長的演員名單。劇組打電話來說：給我一個銀行經理、兩個護士、一個計程車司機。這間辦公室不會挑有趣的人。它挑的是「看起來最像」的那一個，因為劇組要的就是這個。這個說法後來的所有用法，都是從這間辦公室和它送來的那種人長出來的。」
From the word’s history
The name began as a trademark — Central Casting, a real company supplying actors for small parts — and then became an ordinary noun for the work in general.
「這個名字本來是一家公司的商標：Central Casting，專門提供小角色的演員。用的人多了，它就變成一般名詞，可以泛指這一行，也可以拿來開玩笑說某人「像被挑來演的」。」
Wiktionary, "central casting"