The Heart Is a Lonely Hunter sure has a lot of characters. (Don't believe us? Check out our "Character Analyses" page.) So you might be surprised to hear that the tone is pretty much embodied by one single character: Biff. Think about it: he stands alert, eyes narrow, watching over a scene with a level of removal; he intellectually ponders events, while smothering and containing his feelings. You with us? Good. So keeping Biff in mind, let's check out the various aspects of the novel's tone.
This novel thrives on realistic detail, and definitely doesn't shy away from harsh realities. The narrator tend to tell things as they are, and all the characters pretty much take the world as it is. Sure, they all internalize things differently, but they all maintain a pretty consistent, matter-of-fact tone. Here's an example, when Portia is describing a pretty gruesome event in a pretty unemotional way:
"Our Willie didn't have no knife, so he commenced to bellow and ran about the parlor. Then finally Highboy found Willie a razor and he backed up and nearbout cut this Junebug's head off." (2.3.15)
And that's that.
The Heart Is a Lonely Hunter is pretty much a giant character study. In fact, the narrator too often seems to be spying on the various characters: for example, we first come up on Doctor Copeland while he is reading alone in the dark. Creepy!
To top it off, the characters themselves constantly study each other and the world around them. Biff probably best encapsulates, or captures, this tone with his guarded and observant attitude toward the world. But all of our characters are guilty as charged: they're all watching and waiting, hoping for some major revelation. Good luck, guys, but don't hold your breath.