The narrative technique and the tone are both under the umbrella of the book's writing style. And, since this is basically an unedited manuscript, what we have here is pure, unadulterated, Harper Lee. This isn't a Harper Lee cocktail. This is Harper Lee, straight (or maybe on the rocks)—for better and for worse.
The book is raw and even stream of consciousness at times. It flatly describes feelings, like, "She thought she was dying and she began to scream" (9.8) after the revelation of Atticus. The feelings are so sharp and painful, we have to wonder if Harper Lee is writing a semi-autobiographical narrative.
In fact, some chapters are almost entirely dialogue, as if she has a tape recorder taping her life.
Some of the writing is long-winded. This foreshadowing at the beginning of Chapter 8 is a good example:
With the same suddenness that a barbarous boy yanks the larva of an ant lion from its hole to leave it struggling in the sun, Jean Louise was snatched from her quiet realm and left alone to protect her sensitive epidermis as best she could, on a humid Sunday afternoon at precisely 2:28 p.m. (8.1)
That's a loooong way of saying, "we have a bad feeling about this."
But flaws aside, Go Set a Watchman is a fascinating look into the brain of this famous—and famously reclusive—author. If To Kill a Mockingbird is an atom, Watchman is its nucleus.