I'm not op, but I have had to do this a few times because of the same reasons. You pause, take a breath and kindly say "thank you for the opportunity, but at this time I don't think this is the right fit" and leave it at that. No need to embellish, or add extraneous detail or think you're being awkward because they will do the same thing if they don't want to go further in the process with you. It's just business, treat it as such.
If I’m considering ending the interview, I’ll instead critique it on the spot. It’s more interesting for everyone. If they pass me over, that’s fine - I’m already considering walking out anyway.
“Hey, can I stop you there for a minute? This interview style isn’t really working for me to the point that I’m considering cutting the interview short and heading out. Here’s why …” - and then have that conversation. Some people will take that badly - and that’s fine. But I have no idea what will happen next after saying something like that. And that makes it an interesting direction to take it.
