酒が入ると彼の地金が出た。
Once he'd had a drink, his true character came out.
上品に振る舞っていたが、怒った途端に地金が現れた。
She had been acting refined, but the moment she got angry her real nature showed.
苦境に立つと人の地金がわかる。
You learn someone's true colors when they're in a tight spot.