I don't get up and leave. Usually in most interviews at the end companies ask if you have any questions. I usually try to understand their processes and ways of working.

Usually after this, I take some time just to 'calm down' and think about it, and I usually shoot them an email saying that I appreciate the time and opportunity, but I don't think it will be a good match and wish them luck finding a better suited candidate. I had a few replies thanking me, most just don't reply and I had one (only one) insulting me and my experience and saying they only interviewed me out of pity as my experience wasn't good enough (I had more years (8) of experience in the tech stack they used than the number of years the company existed!:)) Was from a Who's Hiring thread here actually :)