How do you write a character's internal monologue?

Question

I've seen two main methods of writing a character's inner monologue

One is to use italics
Dean is being silly thought Sam

Another is to quote the text
"Dean is being silly" thought Sam

Or you could avoid it altogether and simply relate what the character thought
Sam thought Dean was being silly.

Personally I don't like to quote thoughts as it can lead to confusion between their speech.

What is the correct way to indicate that you're quoting their internal monologue as opposed to your characters' spoken word?

Answer 1

I use italics, I find it is the clearest way to define thought as different to speech, and denoting actual thought as a form of dialogue can help draw distinctions between actual thought and narrative.

In third person narrative it is common to write from the perspective of the character in question, and colour the tone of your writing with the way they think. As an example I'd refer you to the First Law trilogy by Joe Abercrombie. Each chapter is devoted to the perspective of a particular character, and each is written in third person limited perspective (so you're effectively inside the character's head.)

The inquisitor is prone to self-analysis, so even though the narrative is effectively coloured with his way of thinking he also thinks as dialogue — in italics — as a way of differentiating those thoughts. Conversely the barbarian Nine Fingers basically never thinks, his narrative is more straight forward and inner dialogue is rarely — if ever — used.

Most of the authors I read do the same so it's certainly a strong convention, but as PraveshParekh says, all are viable options. So while there is no “correct” way, if you are going to use inner dialogue then I believe that sticking to italics — and convention — helps avoid confusion.

Answer 2

In the books I read (mostly SF & F and YA) the thoughts are given in direct speech but not marked up.

Here are the first six books I randomly pulled from my book shelf:

1. Robert Charles Wilson, Bios

   "Even so. It's different, isn't it, when the landscape is alive under you?"
   Alive, Zoe thought. Yes, that was the difference.

2. Kim Stanley Robinson, The Wild Shore

   "We'd be able to [...] do what we pleased."
   And not what your father tells you to, I thought to myself. But I felt the pull of what he said, I admit it.

3. Sarah Dessen, Along for the Ride

   Oh, dear, I thought. Out loud I said, "Oh, right. Of course."

4. Gene Wolfe, Home Fires

   Goodbye, Mort! Sometimes I see you in my dreams.

   The only counter-example is:

5. Suzanne Collins, The Hunger Games: Mockingjay

   a.

   I killed you, I think as I pass a pile. And you. And you.

   b.

   The remaining eight hundred or so are refugees in District 12 – which, as far as I'm concerned, is the same thing as being homeless for ever.
   I know I shouldn't think that; I know I should be grateful [...].

   As you can see, Collins uses italics for some few thoughts, but not for others. In fact the whole novel, told in the first person, consists of the interior monologue of the protagonist, Katniss. The italic parts within this interior monologue appear to be more outward directed (what Katniss imagines she might have said), while the non-italic parts are everything else that goes on in Katniss' brain: what she thinks to herself.
   Here is a third quote from Collins, where this distinction becomes more apparent: again the unspoken words are directed from a speaker to a listener, but here it is not Katniss who "speaks" them in her mind, instead she imagines hearing them:

   c.

   Positioned on my dresser, that white-as-snow rose is a personal message to me. It speaks of unfinished business. It whispers, I can find you. I can reach you. Perhaps I am watching you now.

I pulled out some other books, too, but those were told in the third person and did not have any interior monologue.

Other uses of italics:

• L.E. Modesitt, Jr., The Ethos Effect
  Here italics signify (technical) telekinesis between different persons:

  "Right now, that's all I know, but we'll keep you informed as we can. That is all."
  Weapons status? Van snapped across the shipnet to Lieutenant Michael.
  Tenty-one torps left, ser.
  Thanks, Weapons.

• Gene Wolfe, Home Fires
  Here italics signify voices on the telephone (in quotation marks), Spanish or Latin text (in the dialogue or in the narration), and names of ships.

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Overall, marking up thoughts with italics feels like an amateurish device to me. Wilson, Robinson and Wolfe are writers of the highest order – and they don't use them! In all examples (even including Collins) the distinction between thought and non-thought is unmistakeable without italics, just by the grammar of the sentences and the explicit statement of "I think/tought".

Without italics (as markup for thoughts – or shouting) the text appears more elegant and of higher literary quality.