As someone at a major tech company that is routinely involved in interview loops, I spend at a minimum 25-30 of the 55 minutes I have asking about specific items on the candidate's CV. When white-boarding, I'm interested in two things: a) can they actually write code, and b) how well do they ask questions and collaborate with me when working through the problem.

That said, having gone through loops from the other side, I know this isn't always the case and that your mileage may vary.

Well, shit. I had a whiteboard interview earlier and I was silent while working through the problems. I had a lot things going on in my mind but hadn't taken the time to verbalize them at all.

If the only thing the interviewer has to judge your skill on is what you wrote on the board, that's what will be used. When I'm interviewing, I'm trying to gauge how they approach a problem, how well they can reason through a solution, whether they see the assumptions they're making and the impact those have on the solution, what the limitations are of the algorithm they came up with, etc. It's really hard to see most of this if the candidate doesn't talk at all.