Awesome and complex topic idea!
I think pretty much the same way I converse, but much faster (or about the same as my reading speed, but I dunno) and with much less holding back. I'll even pause with "uh"s and "um"s, or change thoughts midthought (like midsentence). I think I use more run-on sentences when thinking. When I'm reading something, I'll pause to mentally "say" a comment, often with an actual facial expression—kind of like, "Wait, what?" with a confused look. I even add lots of interjecting thoughts between thoughts, as you'll see by my extensive use of parentheses.
Depending on my frame of mind, my thoughts range from very coherent (most often while I'm typing/writing), to a random, incoherent jumble in which I struggle to convert it into something others would understand. (The latter clause was a half-decent example; it still sounds awkward to me.) Sometimes my thoughts are complete sentences, other times fragments. Even if each sentence of thoughts is coherent, I struggle organizing them into paragraphs while writing. Coupled with perfectionism, all of these reasons explain why it takes me so long to write anything.
I don't often talk to myself, even in my head. Whenever I do, it's usually passive exclamations, such as, "That was stupid," or "Shoot!" Sometimes it's in the first person (and typically those are short commands or something like "I'm gonna be late!" or "I wonder if..."). If I ever think to myself in the second person, it's rare. I don't think I ever think to myself in the third person. (Interestingly and ironically, I often dream about myself in the third person.) I pretty much never talk to myself out loud. That seems really weird to me. The instances I do are when I'm trying to quickly ingrain something in my mind (memorization) or when I'm counting—especially when there are lots of distractions around me.
Oftentimes I mentally rehearse things before I say them or write them down, typically to help me remember something and to organize them into something coherent. Unfortunately a lot of the time these thoughts never get around to actually being expressed. Sometimes thoughts suddenly rush to me (like an epiphany), and I need to express them before I forget. This is usually my process when I write poetry (which doesn't happen often), but I tend to do the same thing when I wake up after a dream, but am still lying there with my eyes closed. After sorting out the dream in my head, I quickly have to write it down (or tell someone) or I'll quickly forget it, only remembering random bits and pieces, if anything.
At times, if a thought enters my head which I don't like, I'll close my eyes and ever-so-slightly shake my head (as in, barely visible) to clear the thought from my mind, kind of like erasing on an Etch-a-Sketch. Other times I'll actually "correct" my thoughts, much like going, "No, wait..."
Other times my thoughts aren't actually words at all, but rather feelings or emotions. Sometimes feelings are accompanied by thoughts such as, "Ooh, this is pretty," or "Eww!" or "Wow," but not always. Since it's very hard for me to say something (other than "uh huh") while thinking something else (with words), if I'm speaking and my thoughts are different, they'll just be feelings or intuition instead. One example is if I'm singing to a song I particularly love, I'm speaking the lyrics (and therefore thinking them), but I'm also thinking, "I love this song," without the words actually formulating in my mind, so it's actually just a feeling/emotion, if that makes sense. Another emotion-thought I get is after I say something particularly clever. I don't deliberately think the words, "That was clever!" but it's implied by the feelings. Or perhaps everything I've said about emotion-thoughts is inaccurate; maybe I do think the actual words, but it happens so quickly I'm unaware of it.
All in all, though, even though I think in words a LOT, most of the time my thoughts are visual images, like video clips or photographs. Usually they're kind of blurry, rather than crisp. Sometimes VERY blurry. Sometimes other senses trigger verbal or visual thoughts, and vice versa.
Now if you want to get into the process of what my dreams are like, that's a whole different story. Or maybe an expanded story. That would make a cool topic, actually.
