Interior monologue is a stylised way of thinking out loud. (Technically: thinking ‘on the page’.)
Some people call it ‘internal’ monologue. This is the same thing.
Unlike stream-of-consciousness, an interior monologue can be integrated into a third-person narrative. The viewpoint character’s thoughts are woven into description, using the author’s own language.
This is the essential difference between interior monologue and straight narrative:
Straight Narrative = the narrator talking (You know ‘the narrator’ — that made-up character who sounds like the author — but please don’t mistake authors for narrators — not all authors are crazy axe-wielding, mentally unstable murderers, unlike many of their narrators.)
Interior Monologue = a character talking/thinking, using words specific to that character, making assumptions, mistaken judgements, conclusions RIGHT FOR THAT CHARACTER.
If interior monologue is done well, you won’t even notice it’s happening.
Stream of Consciousness Narrative Technique
Like interior monologue, stream-of-consciousness is another stylised way of thinking out loud.
It is the 19th and early 20th century version of what has become ‘free indirect style/speech’. (A style of third-person narration which uses some of the characteristics of third-person along with the essence of first-person direct speech.)
There’s a lot of interior monologue in stream-of-consciousness but the difference is, there’s no punctuation to mark it out as such.
The terms ‘stream-of-consciousness’ and ‘interior monologue’ are used interchangeably by some — but stream-of-consciousness refers more often to a first person narrative which mimics the jumble of thoughts, emotions and memories passing through a character’s mind. (That said, interior monologue is not necessarily written in first person.)
Stream-of-consciousness tends to be less ordered than interior monologue. That’s because consciousness has no beginning and no end — thoughts flit quite randomly from one thing to another.