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.
(..) At least you can talk. - Who are you? - It doesn't matter who we are. What matters is our plan. No one cared who I was till I put on the mask. If I pull that off, will you die? - It would be extremely painful. - You're a big guy. For you. - Was getting caught part of your plan? (..)2012 The Dark Knight Rises
(..) gets to stay on my aircraft! Who paid you to grab Dr. Pavel? He didn't fly so good! Who wants to try next? Tell me about Bane! Why does he wear the mask? A lot o' loyalty for a hired gun! Well, perhaps he's wonderin' why someone would shoot a man... before throwing him out of a plane. (..)2012 The Dark Knight Rises