Am I correct in believing that a writer should italicise a character's private thoughts?

any help appreciated!

It is not a hard and fast rule, Gillian, but I have seen it done in several novels, and I prefer it to seeing it placed in quotes.
But just as many novels don't bother using anything to highlight thoughts in the text.

- NaomiM

I don't use quotes for thoughts at all. Mostly I don't italicize them, except for the very occasional more immediate/present tense thoughts, but that's rare. But I'm not published, so that may be completely wrong.

I would add that I have not seen it used for large chunks of thought.
Only used to separate thought from dialogue, and then only sparingly, as it does disrupt the flow somewhat.

- NaomiM

The convention is nowadays that only speaking aloud is put into speech marks, and thought is just in normal roman. You do come across books with thoughts in italics, but I always think it looks a bit odd, sort of over-emphatic. Personally the only time I use them is when a character is remember something that someone else said.

Emma

Thanks for the feedback - appreciated!