A man who lives apart, 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, of 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 under all circumstances withhold himself from those in whom he should naturally confide.
Aware of the overwhelming temptations which assail a man permanently isolated in his deceit, Leamas 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 enquired anxiously after the health and prosperity of characters he had created. Similarly Leamas, without relinquishing the power of invention, identified himself with what he had invented.
— John Le Carré, The Spy Who Came In from the Cold