Much of the narrative is told in the third person omniscient style. With this approach, the narrator is able to convey all of the feelings and unconscious thoughts and desires of the characters, really fleshing them out and making them similar to complicated human beings. The third person style allows the narrator to move through time and space, so we can see Joe Christmas when he was five years old, stealing toothpaste, but we can also get to know him when he's a young adult, drifting from town to town.
In addition to third person technique, we also get a lot of the characters telling their stories and back-stories through first person dialogue. Lena Grove tells her story to Armstid in Chapter 1, for example. Similarly, Mrs. Hines tells her story to Hightower and Joanna Burden tells Joe Christmas the history of her family, saving the narrator the trouble of doing so. This mixes things up a bit and allows us to differentiate characters by the way they speak and their perspective on the world. It allows characters to speak in their own language, giving the stories more originality and variety than they'd have if only one narrator delivered them all to us.