John Le Carré said this quote

“ A man who lives a part, not to others but alone, is exposed to obvious psychological dangers. In itself the practice of deception is not particularly exacting. It is a matter of experience, a professional expertise. It is a facility most of us can acquire. But while a confidence trickster, a play actor or a gambler can return from his performance to the ranks of his admirers, the secret agent enjoys no such relief. For him, deception is first a matter of self defense. He must protect himself not only from without, but from within, and against the most natural of impulses. Though he earn a fortune, his role may forbid him the purchase of a razor. Though he be erudite, it can befall him to mumble nothing but banalities. Though he be an affectionate husband and father, he must within all circumstances without himself from those with whom he should naturally confide. Aware of the overwhelming temptations which assail a man permanently isolated in his deceit, Limas resorted to the course which armed him best. Even when he was alone, he compelled himself to live with the personality he had assumed. It is said that Balzac on his deathbed inquired anxiously after the health and prosperity of characters he had created. Similarly, Limas, without relinquishing the power of invention, identified himself with what he had invented. The qualities he had exhibited to Fiedler: the restless uncertainty, the protective arrogance concealing shame were not approximations, but extensions of qualities he actually possessed. Hence, also, the slight dragging of the feet, the aspect of personal neglect, the indifference to food, and an increasing reliance on alcohol and tobacco. When alone, he remained faithful to these habits. He would even exaggerate them a little, mumbling to himself about the iniquities of his service. Only very rarely, as now, going to bed that evening, did he allow himself the dangerous luxury of admitting the great lie that he lived. ”