Her anger and resentment were hidden behind a 'mask' of friendship. [=she pretended to be friendly but was really angry and resentful]
('American football') The lineman grabbed the running back's 'face mask'.
Doctors wear surgical 'masks' in the operating room.
(..) I need your help. Now, it's very important that you remain calm... ...and you do not turn around. - Remain calm. Do not turn around. Behind you in the casino are the Gas Mask Bandits. Oh, my God! The Gas Mask Bandits! - All Europe is looking for them. - How can I help? - Just give me your coat. (..)2006 The Pink Panther
(..) So the chief inspector had a larger one made. You'll find that Paris can be a very political place. Yes, politics. Where greed wears the mask of morality. That's good. Did you say that? Yes, I did. - Could you help me down? - Oh, yes, of course. Oh, yes. Here, just... Perhaps... (..)2006 The Pink Panther