Morrison uses a stream of consciousness style in order to capture the muddled thoughts and feelings of Sethe and Denver and Beloved. Sethe's thoughts and feelings once she realizes that Beloved is actually her baby daughter are not logical or ordered. Instead, she seems to feel so many things all at once that stream of consciousness makes the most sense. In part two, in the section Sethe narrates, she moves from her memory of killing her daughter, to why she was so protective of her breast milk, to her desperate wish that her daughter will understand why Sethe did what she did, to Halle, to the garden she plans to plant, to Mrs. Garner, to memories of being whipped, to Sweet Home and her escape, to the first time she saw Beloved, and so on and so on. The past and present run together because that is how they are in Sethe's head.
It is similar in the next section, which Denver narrates. For her, past and present don't run together, because she doesn't have "rememories" that affect her present, but her conflicting feelings about Beloved and her mother seem to layer over one another. She loves Beloved, but she also seems to recognize the threat Beloved poses to her mother. Denver wants to protect her mother from Beloved, understanding that Beloved strangled Sethe in the clearing, but she also fears that "Maybe it's still in her the thing that makes it all right to kill her children." So, she wants to protect Beloved from Sethe as well.
Similarly, in the next section, Beloved's thoughts and feelings are depicted using a stream of consciousness narration. Beloved seems to mix up past and present, referring to memories she cannot have (like being on a slave ship) as well as describing what it is like to be dead and buried underground. She seems unable either to recognize or articulate what happened to her or when it happened, and she seems to want to become one with her mother—seeing herself as disjointed and horrifyingly separate. Past and present blend together for her, too.
In the last section narrated in stream of consciousness, all three individuals think and feel and talk at once. Therefore, their voices overlap out of necessity. They all want things, and their desires build and conflict.