At the beginning of chapter 11 ("Hidden in Plain Sight") there are two minor interactions that need to be completed. The first interaction is a woman who will sell you an apple. Once acquired, Nate will start eating it. before he finishes, you need to interact with a man with a lemur on his shoulder (He will be further down the road from the woman, down a small flight of stairs). After you finish the interaction with the lemur, continue down the street, and the lemur will follow you, climb on your shoulder, and steal the apple, unlocking the achievement.