"Butter wouldn't melt in his mouth" is a phrase I associate with the WWII generation - my mother would say it (Canadian) , and so would my mother in law (American). It goes along with such phrases as "smiling like a cat in cream" and "letting the fox in the henhouse". Maybe it is more rural? I'm not a big Soprano's fan, so I can't speak to that.
I used a variation of the phrase "if wishes were horses beggars would ride" (in the HP universe that would be "if wishes were thestrals beggars would fly"). My beta reader, who is in her teens didn't pick up on the reference, although her parents apparently did when she asked them.
Its not a very modern phrase, but the HP universe is not a very modern universe, is it?