If we wanted to get any closer to Dicey, we'd have to actually jump into her skin.
That's right—the narrator stays in Dicey’s head throughout the book, showing us the action and the other characters through her perspective. We see both Dicey’s behavior and her thoughts, so even though she’s not telling us the story in first person, we’re constantly engaged with her.
For example, take this quote from when Dicey learns her mother is dying: "Dicey felt as if she was broken into pieces and didn’t know how to gather herself together again. She was angry at herself about this" (10.11). She’s not telling us her feelings in her words, but we still feel them with her.
It's a little trick the fancy lit crit types like to call free indirect discourse. When authors use free indirect discourse, they're slipping into the character's consciousness, while still writing in the third person. In other words, we're reading the character's thoughts and feelings, but we're still reading the third person. Like so:
And they lived happily ever after.
Not the Tillermans, Dicey thought. That wasn't the way things went for the Tillermans ever. She wasn't about to let that get her down. She couldn't let it get her down—that was what had happened to Momma. (1.1-2)
Right there in the first couple paragraphs, we jump right into Dicey's brain. Who else would call Liza Tillerman "Momma"? We can imagine her saying these words, and yet, she's not saying them. A third person narrator is.
Because Dicey is somewhat of a dispassionate character who has trouble voicing her emotions (or even knowing what they are), Voigt’s choice to write the book in the third person allows us to go deeper into Dicey’s brain than we could if we had to wait for Dicey herself to tell us everything. Imagine if this book were written in the first person. There's no way we'd get all the scoop we get, right?