The words of their mouth were smoother than butter, but war was in their heart: their words were softer than oil, yet were they drawn swords. Do you know someone like that?
It looks like is smoother than butter that ulterior motives behind their words. It's always easier to talk to them, when you need a pick me upper is always there.
No one is smoother than butter than you, you manage to find a way to your close one's heart without making an effort. When in need to negotiate, barter, bargain, all your friends know who to call because you are smoother than butter.