Behind the curtain

A term likely originating from stage theatre, to look "behind the curtain" or "behind the scenes" means to gain an insight into how something works/operates beyond what is normally presented to outsiders.

A popular example of its use would be Frank Morgan's portrayal of the Wizard of Oz in the 1939 movie of the same name; "Pay no attention to that man behind the curtain!". By saying this, he's trying - unsuccessfully - to persuade Dorothy and her companions to ignore how the spectacle of the Wizard is actually created; to ignore that the magical wish-granting Wizard is just a man operating an array of special effects.

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закулисье - это не только в театре, но и по жизни: внешняя сторона (зачастую - имитированная/фальшивая/неполная) противоречит сущности вещей/событий (которая не всегда лежит на поверхности)