He puts on an act in front of the camera, but behind the scenes wears a completely different face.
Tips
usage
Strongly negative - used when someone's display of feeling (sympathy, modesty, righteousness) is judged to be theatrical and insincere. Note 惺惺 here means 'feigned / pretentious', not 'wise' (as in the unrelated phrase 惺惺相惜).
mistakes
Don't confuse with 惺惺相惜 ('kindred spirits cherish each other') - same 惺惺 character but opposite polarity. 作态 ('put on a pose') is what makes 惺惺作态 negative.
