One of Sartre's famous concepts is that one's existence precedes essence, that your "essence" or sense of self is defined by the choices you make as you exist.
Kierkegaard expresses the futility of making choices, that whatever one chooses in life is equally meaningless at the point of death. Freedom from this conundrum requires a "leap of faith" which radically surrenders this notion of meaninglessness to a conviction of a morality and ethics outside of societal construct (in Kierkegaard's case, a trust in God, but not the religious institution), a defiance of this apparent human and societal reality.
Sartre describes the phenomenological consciousness, in which absurdity is found in the acute awareness of the grotesqueness of physical reality: you notice the light interacting with certain surfaces in certain ways; what makes this tree stump THIS particular stump? Why the harsh coldness of a table edge? How is this seat interacting with your own physicality in this particular moment in time?
Sartre further expresses the notion of existential "Nausea" in the awareness of an entropic existence, where even in death one's decomposed body is, in one way or another, continuing to exist and affect everything it interacts with. You could never truly cease to exist, and your choices make an eternal impact on whatever you touch.
In his famous Notes From the Underground and The Double, Dostoevsky speaks to notions of vanity and futility in the sense that everyone is endlessly striving for an unattainable goal, that is, a sense of individualistic significance; any sense of self based on pursuits of worldly happiness, order, or beauty are fraudulent. Dostoevsky describes the concept of “Intertia” in the wake of one’s awareness of this endless striving: while the “Man of Action” deceives himself by always staying busy, his trajectory informed by societal expectations, the “Conscious” man sees the truth of his existence, becoming unable to move forward in any direction. This simultaneously launches a sense of chaos which demands an individual’s volition, to act on one’s own will and not along with the flow of society.