Styling a character’s internal thoughts in italics or with quotation marks depends on whether you are quoting from a source that shows a character’s thoughts, writing a character’s thoughts, or editing a text that shows a character’s thoughts.
When you’re quoting a source, use quotation marks to indicate a character’s thoughts, and make it clear in your prose that you are quoting thoughts, not speech:
Walking home alone one night, Julie seems less concerned about the possibility of real danger and more concerned with the likelihood that her mother will be angry, thinking to herself, “Mother will be furious if she finds out I walked home instead of calling for a ride.” . . .
Published 28 June 2019