Sartre sought to synthesize many of his philosophical arguments with fiction. Yet in a play about "self-deception" and "bad faith" the implicit double entendre of characters "play-acting" to be something they are not and actors pretending to play those characters, perfectly complements Sartre's straightforward philosophical argument. In effect, No Exit is a play about the "devouring" gaze of the other and how it restricts one's freedom, incorporated into the play itself and played out on stage through the gaze of the audience members. The characters constantly look for mirrors in order to avoid the judging gaze of each other, while their failure is played out by the constant stare of the play's spectators.
The play's central themes of freedom and responsibility come from Sartre's doctrine that "existence precedes essence." Sartre believed that human consciousness, or a "being-for-itself," differed from inanimate objects, or a "being-in-itself," since humans have the ability to choose and define their individual characteristics, or essence. But with this freedom of choice comes the absolute responsibility for one's action. The fear and anxiety of this responsibility leads many people to ignore both their freedom and their responsibility by letting other people make their choices for them, resulting in bad faith. This is why Garcin is unable to leave the room when the door opens. He can't handle the responsibility of confronting his decision to flee his country, and thus leaves it up to Inez to judge him and define his essence. Similarly, Estelle does not think that she exists unless she looks in a mirror, seeing herself as others do. When Inez pretends to be her "mirror" and says Estelle has a pimple on her face, Estelle's bad faith causes her to accept someone else literally creating her essence. Both Estelle and Garcin are not only "condemned to be free," but are willing to condemn themselves in order to avoid being free.
This emphasis on bad faith establishes Sartre's underlying argument of the play: "Hell is other people." Using only three people and an empty room, Sartre evokes scenes of utter torture and despair. In effect, Inez can't stand Garcin looking at her because she thinks that he is automatically judging her. Since she thinks that is her own role, she accuses him of "stealing" her face. Garcin's mere existence thus reduces Inez's feelings of autonomy. Moreover, both Garcin and Estelle refuse to let go of their pasts, each "looking" at their friends and loved ones back on earth. They attempt to justify their existence by only thinking about their past experiences: as Garcin explains, his "fate" is the evaluation of his past actions by other people. Inez however, sees her past as meaningless and inaccessible, choosing to exist in the present instead. She insists to the others that "nothing" is left of them on earth and that "all you own is here." Rather than justify her existence in terms of the person she used to be, Inez asserts her freedom to choose her essence in the present, even though she is in hell. She is the only character in the play intent on confronting both her responsibility and her suffering--an essential step is asserting her existence. As Sartre explained, "Life begins on the other side of despair."