
Existentialism is a philosophy espoused by Kierkegaard and Nietzsche, adopted in great measure by the Modernist authors—most notably Hemingway—and followed, if not fully understood, by many people throughout the 20th century and even today.
It’s pretty controversial and very interesting. Its 5 principal aspects have comprised the personalities and driven the plots of hundreds of characters and novels. Many writers and readers alike have found an eerie fascination with and attraction to existentialism. You might too.
#1: Life is Absurd

Existentialists say that they believe that there is nothing lasting or real, no absolutes, no final purpose, or anything worth any effort. This ironic and somewhat disingenuous position is undermined by their own insistence upon its absolute truth. If there is nothing real, absolute, or meaningful, then neither is that claim itself. So, they deconstruct themselves.
Nonetheless—petty semantics aside—believing that life is absurd is a darkly comedic place to start in fashioning your protagonist’s personality. Read “The Underground Man” by Dostoevsky. What a ride!
If a character believes life is absurd, it probably is for her. It’s a self-fulfilling prophecy. She may live in isolation and frustration, never satisfied or happy, since nothing means anything. So why bother to try? She could end up paralyzed by her own absurdity. A walking, talking joke.
Absurdity, after all, is the collision between the rational and irrational. Is it not crazily absurd to try to reason with a lunatic? I’m sure you’ve experienced such close encounters, and they make for delightfully comedic scenes—though not necessarily much fun when you’re actually playing a part in them.
#2: Let It Be, Let It Be

The antidote for such absurdity is the character focusing on just “Being.” Read Kosinski’s Being There. What a book. (And what a movie, starring Peter Sellers in his last role.)
Truth, with a capital T, resides in striving toward, in becoming. People attain meaning in their lives not in stasis, but rather in flux. Change enervates; movement defines.
And yes, the Beatles wrote many existentialist lyrics about many intriguing fictional characters. Eleanor Rigby was “wearing a face that she keeps in a jar by the door.” How about Rocky Raccoon, the walrus, mean Mr. Mustard, the girl who came in through a bathroom window, the taxman, Mr. Kite, Prudence, Rita the meter maid, and Lucy in the sky with her diamonds. Absurd characters, one and all. And all fascinating, all classics—all because they just focused on being themselves, living in their own worlds.
#3: Face the Dread

Is there a better word than “dread” to describe that black hole depression, those moments of awful, utter clarity that life may, indeed, be meaningless? In Conrad’s “Heart of Darkness.” Kurtz stared into the abyss and saw “The horror. The horror.”
Existentialists believe that people must face the dread of existence, popularly known as angst, the German word for “anxiety or anguish.” Dread is an awareness that anything is possible, that insecurity is infinite. If accepted, dread destroys all faith in finite ends and prepares the individual for the infinite faith of “Positive Nothingness.”
Characters in the death grip of dread palpitate with authenticity. Try to author such a character. Authorize him with a paradoxical positivity in his nothingness. He knows he’s nothing, but he’s down with that. He can cope—that is until he implodes or explodes.
#4: Make Conscious Choices

Taking charge of your meaningless existence by making deliberate, decisive choices is another strategy characters employ to create a meaning in life when they believe there isn’t one.
Note again the deliciously paradoxical nature of existentialism: only through choosing do we define and construct our individuality. Though all is absurd, meaningless, and dreadful, one must move purposely through life, not drift through it. By choosing, we create our “selves.”
#5: Commit or Be Committed

Finally, the existentialist must make commitments or go utterly insane. It’s Orwell’s doublethink that Winston Smith couldn’t quite grasp, and so he was defeated and committed to a life of dreadful meaningless. One simply must accept the pointlessness while refusing to be pointless himself. That’s the point of life: effort and accomplishment become the reasons to exist.
This formula for survival is Sartre’s “Doctrine of Engagement.” Talk is cheap; there’s no individual reality except in deeds and actions. That’s all that means anything—that is if anything means anything. Said Sartre, “Freedom is responsibility.” Have your characters mull that one over awhile.
Ultimately…

When the absurdity of life is recognized, just being alive is enough, the dread has been stared down, choices made, and responsibility assumed, your existence claims a value in and of itself. In the end—our only friend, saith the Lizard King–nothing else makes sense or is real except existence itself.
If you fashion your novel with these aspects of Existentialism woven into it, you’re likely to end up with quite compelling characters and a fascinating story. Once you’re done, let the highly skilled PhD fiction editors at https://edit911.com have a look at it and help you maximize its quality and chances of publication.