Hi, I'm new here but I find this topic very interesting so I want to contribute to the discussion!
I was living in a dormitory when I heard about people living with a narrator inside their heads that talks CONSTANTLY and I was shocked. I assumed it was something rare, so I had to ask others. I was living with 5 people at the time, and (out of 6) only 2 of of us were "non-narrative" types!
It is super rare for me to think in words or hear a monologue when I'm thinking. (Unless it is a memory but that always comes with images, I will explain this later)
When I do think in a monologue, that mostly happens when I think about thinking in my own voice, if that makes sense? So I believe this means I am capable of it, I just don't use it.
Something that is also rare for me but happens more than any kind of monologue, is when a song is stuck in my head. There are people who complain about this more frequently than I do and it also lasts longer for them. This makes me wonder if this could be because they are monologue-type people?
Now I will try to do my best to explain how my brain works. I have different ways of thinking:
1.The most basic one is when there is no obvious source of thought you could examine like a voice or anything concrete, the thoughts are just there as conclusions. I can describe it like when you use your reflexes, you don't experience the train of thought itself (or maybe you do, for me it's just pure action) you just "know" what to do and execute it. (I have fast reflexes by the way) This is what I do automatically, especially when I actively pay attention to my surroundings. I also use this kind of thinking when I draw something for fun and there is no concrete, visualized idea yet to work with. I never understood people who asked me what I'm drawing, and they were confused when I told them that I had no idea until it "appeared" on the paper. I can draw a face or a house without knowing what it is while I'm drawing the first line. It just happens, it comes from a place I can't describe in any other way than complete darkness. I also use this way of thinking when I read a textbook and it discusses mathematics or something theoretical but not very imagination-inspiring. I focus on understanding and making sense of the information and it is like a transaction with this "dark space" inside my head. I try to store and access knowledge like packages of information but I can't hear or see them. It is like waiting for a database to send a response back to my query, I don't see how it does it or where or what the data is, I just make the request and it responds. When there is a delay, I get frustrated. Like when I forget a word, I just can't access that information because I have no response. Once I tried to remember a password for an account I had on a website. I gave up after a few minutes and I'm not kidding, two weeks later in a random moment when I was doing something completely unrelated I just received the answer. I think this illustrates this database-like behaviour well.
2. Sometimes when I'm either bored (on a train or bus for example and I'm too lazy to play sudoku on my phone and simply looking at what people do is not enough) or I try to imagine something in the real world, I can see both the "real life" and the "mental image" on top of it where I changed something. Like when I try to imagine if a new piece of furniture will fit in somewhere. Or I imagine someone slipping on a banana peel (the idea itself comes from the "dark space" and my imagination takes the idea and creates the image). This kind of thinking is similar to what phones do when you use an AR app with your camera
(My brain can surprise me with very random ideas when I'm in a creative mood, it is very entertaining)
So in the first two examples, there is no narrative or any kind of voice at all. Even if someone would try to force me to think in sentences all the time I would shut up very fast because I would have to concentrate on it to happen. I would zone out and swim in my abstract space of knowledge instead.
When I'm in a coffee shop and I'm thinking about what to order and I have to use a different language, I might "hear" a single word or two if I'm not sure about the knowledge I gathered inside. I try to find out how a certain word sounds like and I mentally "practice" it. It is a very quick process tho, because I would rather pronounce it wrong than spending too much time thinking about it when there is no obvious answer.
3. When I completely enter my head and I don't pay attention to my surroundings at all, I usually work with images or entire movies. This happens when I focus on a memory (with voices added to the mix, if there was a dialogue), try to imagine a scene from a fiction book or a 3D geometric shape to solve a mathematical problem. I also use this when somebody tells me about something that happened to them. My imagination tries to fill in the gaps when their description is not complete, and I ask questions for clarification, based on the image I started to build. Friends sometimes laugh and don't get it why I ask something specific when they said something very general. I have no idea what these decisions are based on because it all comes from the "dark space", but sometimes it is logical. Like, if somebody talks about a chair I assume it has four legs. This is also why I misinterpret things sometimes, I take some details for granted what comes from "inside" and not "outside". This way of thinking is very similar to using a VR app.