I'm talking about a situation where you have narration, dialogue, thoughts, and e.g. a second character inside one character, like if the character was possessed by a demon that now and then spoke to the character inside his head. How would you format the demon's speech? Cursives? Cursives between quotation marks? CAPS (like Pratchett does with Death in Soul Music)? A separate paragraph in bold or a different font (like in Hopkinsons' 'The Salt Roads')? I've tried doing it like I do thoughts and like I do dialogue, but both are confusing because the first makes it look like the character's thoughts, the second like one of the two or several people partaking in the conversation said it outloud instead of the demon speaking inside one character's head. I haven't yet tried caps or separate paragraphs/bolded/different font because I'm worried it would be an MS formatting no-no that would have all agents, publishers, and fellow authors chasing me with torches and pitchforks.
You don't do it with formatting, you do it with the surrounding narrative context. The only difference is that with the character's own thoughts you don't enclose them in quote marks. An other entity's voice inside the head should be treated exactly like spoken dialogue. Writing is done with words and punctuation, not with fonts or graphics. And don't be distracted by fancy fontery used in published books - that's embellishment during typesetting, and has nothing to do with good writing.
Sticking with the dialogue option, then. Just wasn't sure if there was a common consensus in the writing/publishing community regarding this (and, as you said, looking at published novels for guidance, especially from famous authors who can do whatever they want, is hit and miss). Thanks.
The demon in a person's head has to have a different voice from the person, there's no question about that. And I would want to use proper punctuation, it makes sense to me that the demon's thoughts would be in quotes while the individual's would not be. But you get into some tricky territory when the two internal beings are essentially one person with a psychosis rather than one possessed by an external being. I would think the careful use of tags is needed to make it clear where the thoughts are coming from. Since you've made it clear in the past what you mean by fancy fontery, let's just get it out there. I have no opinion one way or the other whether one would italicize both being's thoughts, or just the possessed character. Obviously the choice not to italicize either is always an option and perhaps a simpler one without doing a lot of research to find the more typical conventions. Style choices for internal dialogue are not about embellishment and not mutually exclusive to good writing. One can use less tags with italicized internal dialogue. It's simply a preference. So in the case of multiple character's thoughts inside one head, I would think tags would be more important and more practical. Internal dialogue italics would probably be confusing.
I was just reading a Josephine Tey novel (The Singing Sands) in which the main character was essentially debating with himself, and the debate was formatted exactly like a conversation between two separate characters. "You fool!" said that inner voice, as he was climbing into the London plane at Scoone. "Giving up even a day of your precious leave to hunt will-o'-the-wisps." "I'm not hunting any will-o'-the-wisps. I just want to know what happened to Bill Kenrick." And so on, for another page and a quarter, and again a couple more times in the book. It worked fine, and this character was perfectly sane and un-possessed--I knew that both sides of the conversation were the same person, but it wasn't a problem.