A lot of writing is basic nuts and bolts stuff. One of those nuts is subtext. What is subtext? Well, my glorious chickens, Wiki states that "Subtext is content underneath the spoken dialogue. Under dialogue, there can be conflict, anger, competition, pride...or other implicit ideas and emotions. Subtext is the unspoken thoughts and motives of characters—what they really think and believe."
Basically, a writer spends a lot of their time describing how characters are standing, fidgeting, not fidgeting, glaring, not glaring, etc. in an effort to not simply use obvious and often quite clunky dialogue tags. So instead of "he said nervously" we might go with "he said, his eyes darting toward the window". Or instead of "she said reassuringly" we might go with "she said, putting her arm around his shoulder".
Subtext is an important craft element for many writers-in fact, you could argue that beautifully handled subtext is one of the main strengths of a lot of American fiction (I'm thinking of Ray Carver, Hemingway, Lorrie Moore). In The Art of Subtext: Beyond Plot by Charles Baxter we get a tight little book of craft essays revolving around subtext and its offshoot subjects.
Baxter states that "any thought considered unthinkable qualifies as subtext", which immediately makes one think of classic plays like A Long Days Journey Into Night, A Doll's House, Death of a Salesman, A Streetcar Named Desire, etc. Really, any drama that is propelled by a supercharged idea (or set of ideas) that is either never directly stated or not stated until the narrative's denouement could be considered mainly powered by subtext, which, now that I think about it, is almost all drama.
Among other subjects, Baxter talks about how modern folk have gotten really good at filtering out various ads and how that filtering has crept into our daily interactions with each other (and how showing that can be useful in fiction-people can often talk past each other to both comic and tragic effect). Also, when compared to fiction of days past modern fiction tends to negate the face-we used to get pages of face/body description yet today most authors either skip physical description entirely or give it only a few lines (I myself try to nails my character descriptions in three sentences, max). Baxter states that Don DeLillo, postmodernist that he is, doesn't seem to believe the human face is really important at all (think of Mao II and the mass wedding service in a stadium). Yet if we lose the face, we lose a key element of subtext!
Baxter also talks about having to fight your normal urge to avoid conflict in real life in an effort to bring conflict (which hopefully will itself lead to a revelatory moment) to the page. I found this particularly insightful as a lifelong resident of MN and after teaching fiction and trying bring out conflict in the work of my students.
People who have practiced good manners and conflict-avoidance all their lives have to remember to leave those habits of mind at the door when they enter the theater of fiction. Stories thrive on bad behavior, bad manners, confrontations, and unpalatable characters who by wish or compulsion make their desires visible by creating scenes...The perennial Dostoyevskian question is, "Do you want the truth or agreeable-seeming flasehoods?" Fiction is that place where human beings do not have to be better than they really are, where characters can and should confront each other, where they must create scenes, where desire will have its day, where all truth is beautiful. Fiction is the antidote to the conduct manual.
Show your characters accurately and piercingly as they are on the surface and their true essence will, eventually, come forth. At least, that's the hope.