Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Probing, Melodramatic
This novel revolves a lot around the split between cold, hard facts and emotion. This divide crops up in the narrative, the themes, and especially in the tone.
Nailing down the tone in this book is kind of complicated, since we are dealing with so many different first-person narrators. But there are still some things all the narratives have in common. Since our man Walt is the chief narrator, we're using his narratives to help us sort out the tone. Walter starts and ends the novel, so he helps set up the tone for the entire book.
Fact Finder
Walt wants the facts, and he wants them bad. Who can blame him, when he's wading neck-deep in mystery? We'd be fact-crazy too, if we were surrounded by as many lying liars as Walt is. Poor dude just wants the truth:
Thus, the story here presented will be told by more than one pen, as the story of an offense against the laws is told in Court by more than one witness—with the same object, in both cases, to present the truth always in its most direct and most intelligible aspect. (1.1.1.4)
Walt sounds a little dry here, but it's for the same reason that legal documents sound dry—he's just trying to state the truth, the whole truth, and nothing but the truth. The lack of poetic flourish helps convey a factual tone.
But there is a lot of emotion simmering underneath here, too, and the sentences gradually start to get a little more eloquent as Walter touches upon the subject of Laura herself… someone who definitely brings out his inner poet.
Drama Llama
Walter's descriptions of Laura can actually veer into melodrama, especially when he starts talking about their star-crossed love affair:
The last word went like a bullet to my heart. My arm lost all sensation of the hand that grasped it. I never moved and never spoke. The sharp autumn breeze that scattered the dead leaves at our feet came as cold to me, on a sudden, as if my own mad hopes were dead leaves, too, whirled away by the wind like the rest. (1.1.10.40)
Whoa. Simmer down, Walt.
But check out how absolutely different from the first quote this one is. From the nothing but the facts, ma'am brusqueness of "with the same object, in both cases, to present the truth always in its most direct and most intelligible aspect" to throwing out super-dramatic poetic phrases like "bullet to the heart," "my own mad hopes were dead leaves"? Walter is starting to sound a little unhinged.
But that's the point of the novel—these incredible events have the power to both derail rational minds and make would-be detectives out of the most mild-mannered citizens.