Some people love to pretend as if they are angels on this planet but the way they react at others back is the worst thing one could do. Why do people pretend?, when they can be straight enough.
Becoming sweet isn't an issue if people are the same at your face and even behind your back.The issue is hypocrisy which should be resolved.
A liar is mostly not caught because they have been lying since there childhood, if one is continuously changing there facial expressions most probably they are lying.