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.”

But if your source shows the thoughts in italics, reproduce the italics and enclose them in quotation marks:

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.”

If you’re writing fiction, you may style a character’s thoughts in italics or quotation marks. Using italics has the advantage of distinguishing thoughts from speech.

Her footsteps echoing across the poorly lit, deserted alley, Julie thought to herself, Mother will be furious if she finds out I walked home instead of calling for a ride. Suddenly, she heard a voice behind her. “Julie! Why didn’t you wait for me?” In the darkness, Kayla, her cross-country teammate, came into focus. She had completely forgotten that Kayla had asked to walk home together.

An editor should follow the house style guide in styling characters’ thoughts. If house style allows it, an editor may follow the author’s preference for italics or quotation marks.