Expression for a "smiling face tiger"

I don't know Chinese but see: 衣冠禽兽

When we hear that about a “wolf in sheep’s clothing”, we know to expect someone who we can’t take at face value, but have to be careful as to what their true intentions may be. As it were, there is also a Chinese saying that means the same thing: 衣冠禽兽 (Yīguānqínshòu).

… Are you wary of sheep lest they be wolves inside?

Lip server/play-actor is exactly who/what you are talking about. Two-timer fits loosely, too. Lip-server is derived from lip service (to just say something but not actually do it. To pretend that you believe a certain thing but not practice that belief. To pretend that there is no problem when there is actually a problem etc.) http://www.urbandictionary.com/define.php?term=lip+service

A play-actor plays the part of a caring, attentive and affable individual while playing is easy, but when the chips are down his true colors become revealed.

Well, every one of the following words should serve well in describing a person that appears to be (as in easygoing, cordial, friendly etc.) what he/she is not: fake, two-face, fraud, phony, double-dealer, two-timer, play-actor, poser, charlatan, quack, sham, humbug, impostor, pretender, masquerader, cheat. These are the ones off the top off my head and there are many others out there. As far as idioms go I could only think of "a wolf in sheep's clothing" (a dangerous person pretending to be harmless)