画猫画狗难画鬼知人知面不知心
[huà māo huà gŏu nán huà guĭ zhī rén zhī miàn bù zhī xīn]
An idiom indicating knowing someone's outward appearance does not equate to understanding their true nature or intentions just like trying to sketch unseen things (like spirits/ghosts). It highlights human complexity that lies beyond what appears superficially visible.