How do you present the thinking inside a character’s head? The old-fashioned way was to tell it: ‘Henrietta thought Edward extremely ill-mannered, etc’, and that’s still a possibility—briefly. But not for long passages. Nowadays we want a more direct experience of thoughts, not being told about them from a distance.
Direct Interior Monologue is when you go directly into a character’s mind, quoting their verbalised thoughts just as you might quote their spoken words. (I used to teach this stuff when I was a university lecturer. Hmm … I didn’t own up to that before, did I? My only excuse is that I always lectured about books more in the style of a writer than an academic!)
Anyway, Direct Interior Monologue developed at the start of the 20th century as a bold new literary experiment. The rationale was that every experience turns into thought, but a sub-verbal stream of consciousness rather than a soliloquy in sentences. So here’s James Joyce, reproducing that stream directly onto the page:
(Leopold Bloom thinks—) This is the very worst hour of the day. Vitality. Dull, gloomy: hate this hour. Feel as if I had been eaten and spewed.
Provost’s house. The reverend Dr Salmon: tinned salmon. Well tinned in there. Wouldn’t live in it if they paid me. Hope they have liver and bacon today. Nature abhors a vacuum.
May the Great God of Literature spare me, but basically it doesn’t work very well. Of course, no one dares to say so about a literary classic, but the experiment failed as experiments sometimes do.
Stream-of-consciousness interior monologue of this kind has never caught on in fiction at large. Very few writers would try to use it nowadays, and they’d be writers with very few readers.
It’s not that the associations are obscure (though they often are), but that this form of direct quotation doesn’t do what it’s supposed to do, that is, capture the true nature of our inner thoughts. Although our thinking is partly verbalised and partly non-verbalised, it isn’t a continuous stream of truncated phrases in this way. It would be hard to say what it is, but not that.