If he is a big enough fool, you can get him to realise the character of the friends only while they are absent; their presence can be made to sweep away all criticism. If this succeeds, he can be induced to live, as I have known many humans to live, for quite long periods, two parallel lives; he will not only appear to be, but actually be, a different man in each of the circles he frequents. Failing this, there is a subtler and more entertaining method. He can be made to take a positive pleasure in the perception that the two sides of his life as inconsistent. This is done by exploiting his vanity. He can be taught to enjoy kneeling beside the grocer on Sunday just because he remembers that the grocer could not possibly understand the urbane and mocking world which he inhabited on Saturday evening; and contrariwise, to enjoy the bawdy and blasphemy over the coffee with these admirable friends all the more because he is aware of a ‘deeper,’ ‘spiritual’ world within him which they cannot understand.
You see the idea—the worldly friends touch him on one side and the grocer on the other, and he is the complete, balanced, complex man who sees round them all. Thus, while being permanently treacherous to at least two sets of people, he will feel, instead of shame, a continual undercurrent of self-satisfaction.
–from C. S. Lewis, The Screwtape Letters