Tāmen liǎ běnlái zhǐshì jiǎzhuāng qínglǚ, méi xiǎngdào nòngjiǎ-chéngzhēn.
The two of them were only pretending to be a couple, but ended up actually falling for each other.
Tips
history
Recorded in Song-dynasty poet Shao Yong's 弄笔吟: 弄假像真终是假，将勤补拙总输勤 - 'fake made to look real is still fake.' The idiom inverts the original: sometimes the fake actually does become real.
culture
A favorite romantic-comedy trope in Chinese fiction and TV: fake-dating, fake-marriage and fake-engagement plots that turn into the real thing - 弄假成真 is the standard label for the genre.
