画人画骨难画心知人知面不知心
[huà rén huà gú nán huà xīn zhī rén zhī miàn bù zhī xīn]
'Easily Draw The Face But Not The Heart - Easy To See Someone’s Face But Hard To Understand Their True Nature.' It's from a Chinese idiom meaning that while it's easy to know what someone looks like, truly understanding them is far more difficult. People might hide their true feelings.