There are no hard and fast rules about displaying a character’s thoughts. It tends to be dictated by trends of the moment or personal preference. The one rule is that once you have chosen a style, be consistent in the use of it.
Below are a set of published conventions for the expression of thoughts.
“Thought and imagined dialogue may be placed in quotation marks or not, so long as similar instances are treated consistently within a single work.”
Oxford Style Manual, Oxford University Press, 2003
“…Italics are also used for emphasis…In fictional works italics may be used more creatively, for example, to convey unspoken thoughts.”
Mitchell & Wightman, Book Typography, A Designer’s Manual, Libanus Press, 2005
(It is worth noting here that there are conventional editorial rules for the use of italics, and I shall cover these in a future post.)
“Some authors have their own system of quotation marks, which they are anxious to retain: for example, double quotes for speech and single for thoughts…Try to persuade your author not to do this, as it can be more confusing than helpful.”
Butcher, Judith, Butcher’s Copy-editing, The Cambridge Handbook for Editors, Copy-editors and Proofreaders, 4th edn, Cambridge University Press, 2006
Placing quotes around direct thoughts is now deemed old-fashioned. Modern convention is to display them without quotes. Italics can be irritating for the reader, especially if used a lot. It’s worth researching traditionally published novels to see how thoughts are displayed. Here are a few examples:
‘Wish they could see famous Harry Potter now,’ he thought savagely, as he spread manure on the flowerbeds, his back aching, sweat running down his face.
Rowling, J. K., Harry Potter and the Chamber of Secrets, Bloomsbury Publishing, 1998
She looked up from her desk and glanced at Mma Makutsi, who was busying herself with the typing of a letter which Mma Ramotswe had drafted, in pencil, earlier on. We must try to help her, she thought. We must try to persuade her to value herself more than she does at present.
Smith, Alexander McCall, The Kalahari Typing School for Men (No. 1 Ladies’ Detective Agency), Abacus, 2004
So then I went back to the clearing he still wasn’t there and I thought Well, I guess he just made up he was coming and he didn’t want pants so bad after all.
Niffenegger, Audrey, The Time Traveler’s Wife, Vintage, 2005
(It is also worth noting that the first two are written in third person, and the third is written in first person.)
My personal preference is to use italics to display thoughts and imagined dialogue, but this will depend on the context, and how much text has already been italicized. 🙂