Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Detached vs. Dark; Melodramatic
When we're reading the playwright's descriptions of characters and places, or the notes about how characters say or react to something, O'Neill's narrator is matter-of-fact and to the point. Like when he describes Seth as "an old man of seventy-five with white hair and a beard, tall, raw-boned and stoop-shouldered." The narrator isn't invested emotionally in what's going on, he's just there to describe it. No matter how awful or horrible, no matter how much it might hit you like a punch in the gut, the narrator stays out of it.
When the characters get to talking, however, that's when things start to heat up. And fast. It seems like no two characters can be together for very long without an argument breaking out or letting loose with something dark and intense, like when Ezra tells Christine that there's something sitting numb in his heart like a statue of a dead man in a town square.
Even when the conversations seem light and airy, it's forced; there's usually a secret being kept. What might pass for tender displays of affection between a mother and her son or a father and his daughter seem weirdly incestuous. The characters bring a whole range of strong emotions to O'Neill's trilogy that the calm and straightforward tone of the narrator does not.
There's a highly melodramatic tone in a lot of the dialogue between the main characters. Glares and accusations fly back and forth in hugely emotional interchanges. War and its horrors are described in over-the-top language. Christine and Orin's discussions are dripping with incestuous overtones. And people are always dropping dramatic hints about suicide or murder. There's non-verbal melodrama as well. Ezra clutches his chest and falls dead with an accusing finger pointing to his murdering wife; Christine faints dead away as the telltale pills roll slowly out of her hand. Nothing subtle about that; it's a play, and it's supposed to be seen.