To the existentialist, actions aren’t just a tool of characterization – they are the only tools of characterization. It doesn’t matter what one thinks or claims or believes. It matters what one does. The best example we have is Garcin. Garcin claims to live the life of a courageous man. He aspires to be courageous, talks about being courageous, and believes he is courageous. Yet he acted in a cowardly manner on earth and makes a final, cowardly decision in hell (to stay in hell and avoid facing his own freedom and responsibility). Because actions are what matter, Garcin is a coward in spite his beliefs/desires/aspirations/claims.
Because we’re working with a play, there isn’t much opportunity for the author to tell us what we need to know about the characters. Often this sort of authorial telling is done via stage directions, but Sartre refrains, preferring to let his characters speak for themselves. And speak they do. Inez’s reticence and Estelle’s denial are immediately clear from their words and attitude upon entering the stage. Of course, much of the play consists of dialogue and debate, all clues as to how to interpret these various characters. When Inez argues with Garcin over his courage, for example, we see that she embodies the existentialist’s point-of-view, while he is embracing bad faith.