butter wouldn't melt in sb's mouth 表面一副正经模样；装老实相

【例句】
Sure, he looks as if butter wouldn't melt in his mouth when he's around people he doesn't know, but stay a while and you'll see what he's really like.
当他在他不认识的人周围的时候，他会装老实相，但是过一会你就看出他真正的样子了。

He looks as if butter wouldn't melt in his mouth,but I'am convinced he is behind all this.
他看来十分老实，但是，我深信这一切都是他挑起的。