I do not consider myself particularly experienced working with spirits (directly, anyhow) but I will tell you what I can from my limited experience. Although, I partly worry it might cause someone to kid themselves.
Ultimately spirits do not really communicate with a voice and words, they communicate in meaning. Some part of you is just a very good interpreter that would make UN officials blush, and simultaneously translates that meaning into words for you. However, not everything fits in just words.
Because of this, IME, communication with spirits will always consist of various layers that are not just words. Any voice you get from a spirit will have other things coming at you other than just words and all simultaneously. How you will interpret all that I can’t say, since I’ve never really talked about it much, or seen it talked about too much, other than the occasional reference to “downloading packets of information”. I know that often I interpret it as alternating “waves” rushing upon me as they speak, often with some kind of corresponding “emotion” that hits me as the different waves crash upon me. On occasion, it may seem like something is being “shot” into me very rapidly.
The voice in your head however, is very well…in your head. It is almost like it is coming from within a hollow echo chamber that is devoid of much else.
They also probably won’t speak with your voice (and I want to say never will, but I don’t want to offend anyone - and haven’t experienced everything there is to experience). You know, that voice that you always have in your head, that tends to sound something like your physical voice. They will have their own voice, more along the lines of what you hopefully experience reading this: https://i.chzbgr.com/maxW500/5762738176/h6CE90515/ All with their own mannerisms and way of speaking.
Of course, the most obvious way you know it “isn’t in your head” (although it’s all ultimately “in your head”) is when you hear their voice with your physical ears, exactly like you would with another person. Heck, the first time I experienced that was with another person - I had my back turned to my mother and heard her ask me a question (“what are you doing?”). I answered. And we were both shocked, because she had only thought that, not opened her mouth. But to me, it sounded exactly like she had.
I can’t speak for Mr. Koetting, but I would presume both, at varying times. In the best of cases, you should indeed see and hear the spirit just like you would another person who stopped over for a visit. But you may not always get quite so lucky.