How does Darl's character develop over the novel?
In my opinion, Darl Bundren is the most interesting character in As I Lay Dying. He is an intuitive, misunderstood by the world and the other characters of the novel and, as such, is labeled insane. Darl's character in the novel develops through the minds of the other characters (as is usual in the Stream of Consciousness Technique). Quite simply, and because of the limited vision of those other characters, Darl's character achieves a range that engulfs the entire spectrum of intelligence from dimwitted to intelligent (in regard to valid attempts at purification) to psychic.
First, we'll look at the family's very limited thoughts about Darl, who is always labeled as either dumb or dimwitted or feeble-minded. There are simple reasons for this label such as Darl's repetition of words, sounds, and seemingly incongruent statements:
Yes yes yes yes yes yes yes.
In this dysfunctional family, Darl is always seen as different or strange. He is ignored and looked down upon. It is Faulkner's use of italics that should show us, first, that Darl is quite different. His memories and his thoughts are shared in this way. A good way to remember this simple idea of Darl is in his description of sleep as being "emptied."
And so if I am not emptied yet, I am is.
This is a perfect example of a statement that the shortsighted family would not understand but we, as readers, receive clarity in knowing that Darl has a battle going on within him. It is a battle between Darl the family member (brother and son) and Darl the person or Darl the soul. It is the latter "character" that causes Darl the most issues, and eventually leads to his internment.
Next, we have to look at Darl's intelligent attempt to purify the household. Why? This is where the family can no longer afford compassion for Darl. Why can we, the reader, label Darl as intelligent? Simple. He knows the real reason Anse and the others are heading to Jefferson. As a result, he tries to purify the farm and the family by burning the barn containing the dead body. The family can no longer afford any niceties toward Carl. They pronounce him insane and have him committed. Let's take a look at two important quotes. The first is from the beginning:
With that family burying-ground in Jefferson and them of her blood waiting for her there, she'll be impatient. I promised my word me and the boys would get her there quick as mules could walk it, so she could rest quiet.
Yet another quotation is from much later in the book that reveals even more:
She cried hard, maybe because she had to cry so quiet; maybe because she felt the same way about tears she did about deceit, hating herself for doing it, hating him because she had to. And then I knew that I knew. I knew that as plain on that day as I knew about Dewey Dell on that day.
It is this quotation that completes the development of Darl's character in the novel because it proves Darl to be intuitive: "I knew that I knew." Not only this, but "I knew ... as plain on that day." Here, again, it is important to note Faulkner's use of italics in the novel. Private thoughts are revealed here. As readers, we begin to realize that Darl has had experiences apart from his body and is, then, gifted with "second sight." Darl "knows" things. Here is where some scholars differ in their account of Darl's character: while one scholar thinks Darl suffers a breakdown and burns the barn, another scholar thinks that Darl attempts purification due to his intuition.
Unfortunately for Darl, he is part of a family whose compassion for him cannot rival his compassion for them. As a result of his knowledge and intuition (or some say because of his breakdown), Darl is committed to the insane asylum in Jackson. Some say this makes Darl the tragic hero of the novel.