It wasn't until C. G. Jung published his book Psychological Types that the terms "Introvert" and "extravert" which he coined came into general usage. But people had always known that such types existed. Authors had been creating introverts and extraverts for many centuries. Hamlet, for example, is an introvert. Falstaff is a good example of an extravert (or extrovert). Introverts like solitude and prefer to work alone. Melville seems to have created Bartleby mainly to show an extreme example of an introvert, long before the term itself was coined by Jung.
Since Jung's time psychologists have been devising tests of introversion/extraversion based on his book. Now they have scales on which to classify individuals from 1 to 100, with 1 being extreme introversion and 100 being extreme extraversioin. Bartleby would probably rank close to 1 on such a scale. Some of these tests can be taken free online by searching for "introverion-extraversion self tests." One such test consists of 81 simple questions, such as "Do you prefer working alone or working with others?"
It is interesting to try to classify literary characters, and even authors, as introverts or extraverts. Holden Caulfield of Salinger's novel The Catcher in the Rye is an obvious introvert, as was Salinger himself. William Faulkner seemed like an introvert, as does his character Quentin Compson in The Sound and the Fury.
In "Bartleby the Scrivener" Melville obtains his effect largely by contrasting the introvert Bartleby with his employer who is an obvious extravert.
As the psychological double of the lawyer who confines his life to Wall Street, Bartleby represents the imaginative side of the attorney. He works industriously until the lawyer puts up a screen between the two. Then, in a very significant three day period, the imagination of the scrivener dies and Bartleby merely responds, "I prefer not" to any requests of the lawyer.
See the essays and criticism section on enotes and also read the responses in the Question Group for more insights.
How miserable it would be to live as unhappily as Bartleby lived. He was discontent and unfulfilled. His office space was cramped and uninspiring--even his window had only a view of a wall. His job, copying the tedious legal contracts others wrote, is not satisfying in any way, either. The passive-aggressive behavior of Bartleby is a reflection of his discontent with his world--a world in which nothing was expected of him but rote, tedious behavior. He simply turned that around and gave it back to them in the form of his life's mantra: "I would prefer not to."
Bartleby could appear to be crazy, but he is quite misunderstood. He is a true artist at heart and is not fulfilled because he is not able to reach his potential as such. He is stuck in an office doing work he objects to doing. He is out of his element. Bartleby is not lazy, nor is he crazy, in my opinion. He is clearly misunderstood and cannot express his feelings and emotions clearly to others. This severely limits his ability to make others understand his true needs and desires. He reacts in the only ways he knows how. He retreats into his own "world" in his mind and will not let anyone into that world. Because he refuses to do this, he ultimately dies after starving himself.