whats the difference between sanity and insanity, as portrayed in the novel?

O K. This is difficult as most of the characters in this novel are just a little loopy. Sanity -vs- insanity. We'll have to look at Darl first. He is the only truly insane character from the novel. He was already mad at the introduction, and the things that happened to the family (mother's death, storm, traveling, ect.) only served to unhinge him. For Darl, the complete entrance in madness came with the burning of his mother's body.

The rest of the characters all have the capability of joining him. Maybe Faulkner's intent was to shadow that fine line between the two states in order that the reader might see how very close together they really are; maybe he just wanted us to understand that there's a bit of madness in everyone.

Jewel resisted and conquered madness because he was the toughest of the group. Vardaman, as young as he is uses actions that some might perceive as mad to understand death........... but he's not mad; he's just innocent and there's no one else to help him. This very dysfunctional can't help itself in the least.

In the end of the novel, Cash talks about Darl and his insanity. “But I aint so sho that ere a man has the right to say what is crazy and what aint. It's like there was a fellow in every man that's done a-past the sanity or the insanity, that watches the sane and the insane doings of that man with the same horror and the same astonishment”. Cash says every man is insane in some way or another, it just depends on what drives them to the brink of their insanity. Addie's death, the trip to Jefferson, and all the other misfortunes in Darl's life led him to reach insanity.