If I had more space, I’d happily expatiate on what’s wrong with this mortifying passage. But two things leap out right away: the gratuitous humblebrag about the speaker’s fancy expertises (sashimi!), and the labored, self-conscious explanation of her narrative context. The author of this tortured self-advertisement wasn’t a born novelist.

But she was a born critic. In works such as “Against Interpretation,” the most casual observation — “People run beautifully in Godard movies” — has a pith and an authority totally lacking in such overwrought, overthought and overwritten novels as “The Benefactor” (“He always spoke across the unbesiegeable moat of his own chastity”). And yet, like certain other distinguished critics — Edmund Wilson, Lionel Trilling, James Wood more recently — Sontag couldn’t resist the allure of being thought of as a novelist. The fact that these writers’ fiction pales alongside their nonfiction raises questions about the qualities proper to each genre — and about why few writers excel in both.

The tools of the critic are, in fact, precisely what you find in that clanking Sontag passage: unabashed expertise, heightened self-consciousness, an impulse to explicate. Wilson’s Olympian knowledge of American and European literature allowed him, in “Axel’s Castle” (1931), to situate deftly — and correctly — both Proust and Joyce in their literary and cultural contexts remarkably soon after their works were first published. In “Notes on ‘Camp,’ ” Sontag’s intuitive feel for a work’s mechanics — how everything from science fiction to the nouvelle vague produces its meanings — allowed her to illuminate the mechanics of culture itself.

But that very erudition and didactic élan can be impediments to the natural flow of a story and the persuasive delineation of character that distinguish good fiction. In an essay marking the 1967 republication of Wilson’s first novel, “I Thought of Daisy” (1929), the British sociologist John Thompson slyly compared two literary Daisys: Wilson’s heroine and her near contemporary, the love interest in “The Great Gatsby.” When we meet Gatsby’s Daisy, Fitzgerald describes her looking as if she “had just been blown back in after a short flight around the house”: a phrase that instantly conveys, as Thompson put it, what a “foolish, sad and delicious creature” Daisy Buchanan is. Wilson’s narrator, by contrast, leadenly informs us that his Daisy is “interesting, attractive, amusing and profoundly sympathetic.” Unsurprisingly, it’s only in Wilson’s foreword to his early novel that his characteristic verve and authority appear — it’s where he can be his true self.

What’s striking is, indeed, how often these astute critics fail to understand their true selves. Much of this has to do with the centrality of the novel in the literary imagination of the 19th and 20th centuries: To be a “writer” meant to be a novelist. “How he will produce and mean something to the world. . . . And how far-far-far I am from being a writer,” the excellent (nonfiction) writer Trilling moaned in 1933, apropos of Ernest Hemingway. “I am a storyteller,” Sontag declared, an assertion her inert fiction never bore out. Today, when the rise of narrative nonfiction and memoir has redefined what stylish literary prose can be, it’s agonizing to see such brilliant writers — writers — hopelessly chasing a chimera: managing, like the narrator of “In America,” to understand so much of what others are saying, yet failing to hear their own voices.