butter won't melt (in one's mouth)

Said of one who appears innocent, modest, or reserved in certain company, when in fact one may have the opposite demeanor. Sure, he looks as if butter won'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. Samantha always affects this demure, butter-won't-melt expression on her face whenever she wants something from you. I know you think that Erin's just pulling some "butter won't melt in my mouth" act, but now that I've spent more time with her, I think she genuinely is shy.