In some works of literature, a character who appears briefly, or does not appear at all, is a significant presence. In Beckett's Waiting for Godot, the character of Godot remains absent for the full duration of the narrative (or lack thereof), yet he provides the impetus for all of the play's themes. Vladimir and Estragon's substanceless rhetoric revolves completely around their fate of waiting incessantly for this savior from the mundane. Godot could be a God, a friend, or a symbol for general enlightenment - Beckett never specifies. Yet the fabled Godot manages to affect the drama's series of action, its theme, and the development of its two characters.
What little action occurs in the nihilistic realm of the two main characters comes about through an infinite wait for the appearance of Godot. A large chunk of Vladimir's and Estragon's nonsensical vocal acrobatics revolves around their quest for fulfillment, which takes the form of this (nonexistent?) entity. This leads to Beckett's realization that all of humanity's day to day tasks come to naught in the larger spectrum of the universe. Godot becomes the centerpiece of this pointless existence through the banal actions of the two characters, such as the argument over a boot and the parodic contemplation of suicide.
This absent character also provides the apotheosis of Beckett's existential philosophies. The work's main theme deals with the futility of modern man's faith in religion, government, reason, and general social institutions. Godot represents a conglomeration of all of humankind's ideological foundations, and the fact that he/she/it never appears, despite the incessant quibbling and superficial efforts of Vladimir and Estragon, suggests that he/she/it, like all of man's institutions, exists solely in the abstract imagination of a select few, never amounting to any practical purpose.
Finally, Godot gives the stimulus (or lack thereof) for the character development of Vladimir and Estragon. The fact that the two characters undergo no change whatsoever over the course of the play - even in the midst of minute changes between the two scenes, such as the appearance of leaves on the trees - suggests that their obsession with Godot and the minutiae of life stunts any possible character growth. The duo becomes an archetype for all of humanity and its trite, hypocritical attachment to its beloved ideology, symbolized by God-ot. The two characters, and thus mankind in general, receive no development due to their slavish adherence to Godot and his/her/its ideals.
Thus, the noncharacter of Godot serves as the barrier to the drama's action, theme, and character development, and the fact that Vladimir and Estragon eschew any semblance of embracing the potential of life in favor of this blind faith demonstrates the pointless, repetitive nature of man's devices.