This question is more complicated than it sounds. Even though Italics are usually pretty reliable to indicate a break with the narrative in general (they may indicate an internal monologue for instance), a lot depends on who has edited the text.Italics are not a form of punctution like commas or hyphens, but a change in font that denote a change in the pattern of expression.
Especially when it comes to Faulkner, publishers and editors have tackled his writings in different ways. I think one can only answer this question in a careful and comparative fashion by looking at the publication of a certain text and by comparing it to Faulkner's original. Most of his publicicized works keep his Italics, but some insert different forms of punctuation ( Absalom,Absalom for instance).
That being said, there is no sure way of reading Faulkner's Italics.
Many times during this short story by William Faulkner, the author uses italicized areas to highlight Sarty's thoughts and feelings. Throughout the novel Sarty doesn't always say what he is feeling because of his sense of loyalty, love and fear of his father. His growing maturity and moral conflicts increase in the progression of the story. Since the story is told from Sarty's point of view, rather than stop and explain each time he is having an internal conflict with the action of the story Faulkner puts Sarty's feelings and thoughts into italics so the reader is immediately aware of what we are reading.