Kierkegaard on life:
In the theater, the play is staged before an audience who are called theatergoers; but at the devotional address, God himself is present. In the most earnest sense, God is the critical theatergoer, who looks on to see how the lines are spoken and how they are listened to: hence here the customary audience is wanting. The speaker is then the prompter, and the listener stands openly before God. The listener, if I may say so, is the actor, who in all truth acts before God.
Sartre used the metaphor of an actor as well:
His movement is quick and forward, a little too precise, a little too rapid. He comes toward the patrons with a step a little too quick. He bends forward a little too eagerly; his voice, his eyes express an interest a little too solicitous for the order of the customer. Finally there he returns, trying to imitate in his walk the inflexible stiffness of some kind of automaton while carrying his tray with the recklessness of a tight-rope-walker by putting it in a perpetually unstable, perpetually broken equilibrium which he perpetually reestablishes by a light movement of the arm and hand. All
his behavior seems to us a game. He applies himself to chaining his
movements as if they were mechanisms, the one regulating the other, his
gestures and even his voice seem to be mechanisms, he gives himself the
quickness and pitiless rapidity of things. [...] He is playing, he is amusing himself. But what is he playing? We need not watch long before we can explain it; he is playing at being a waiter in a café.
As Sartre goes on to elucidate, the unbearable angst emergent from our ephemeral consciousness is often replaced by a "role" — an "act". What's more, as the guests respond to the waiter, he becomes
more convinced of his personality (highschoolers demonstrate this principle perfectly — the outcast becomes more outcast, the bully more of a bully, the clown more of a clown...).
Perhaps Kierkegaard's actor and Sartre's actor aren't as different as they first appear: what if religion is a massive play, where everyone reminds each other of the "reality" of their role, and continues on, encouraged but deceived? How can you possibly tell the difference between the actor and the authentic?
There would have to be some sort of objective reality behind the actions of the individuals (after all, science would have this problem of inbred certanity as well if it wasn't for its attempt at objectivity). A Christian, for example, would have to be completely themself in a way the waiter is only attempting. I doubt you can resolve this in others, but might you answer for yourself?
The existential idea of angst could be approximated as a fear of potentiality. We take on roles to overcome that fear — by limiting our freedom we find a cheap imitation of security. We'd know, then, that we weren't acting (i.e., not deceving ourself and riding on the affirmations of others) if we had freedom without fear.
That's seems like an easy line to draw between naïve and realistic philosophies: there's hope in legalism and asceticism on one side (the
denial of our "nature"), on the other, there's hope in the revelation of our
true self.
(Note: Deriving the qualifications for our essence from Sartre's existentialism, which asserts that "existence precedes essence", is ridiculous. For the above conclusion to be coherent, we would have to reimagine his definition as something about coming to understand an essence previously unkown.)