Changing someone's habits — easier said than done.
Tips
history
From Dongfang Shuo's Han-dynasty 《非有先生论》 (also in 《汉书·东方朔传》): '於戏！可乎哉？可乎哉？谈何容易。' Dongfang Shuo sighs about how dangerous it is to speak frankly to a ruler.
usage
Usually a sentence-final exclamation rebutting someone's casual suggestion, or follows a statement implying 'sounds simple'. Often preceded by 说起来 / 做起来 / 要...谈何容易 for the classic contrast.
