I've read lots of these so it's only right I share my experience today
I arrived at 7 and there were 15-20 in the queue already, about 7:30 a stream of staff came out of the embassy and began putting out tables and boards.
2 women then came down the line checking interview times and separating the later appointments into another line.
As I had seen advised I mentioned I was there for a K1 interview and was moved to another smaller line (I was the 2nd)
Next a man checked my interview letter and passport and ticked me off the list and sent me to security which is like any airport.
A short walk round the building, up some steps and into the embassy where they check the DS160 and give you your number, then into the main room to wait.
My appointment was at 8 and at 5 past my number popped up on the screen directing me to window 15 where a nice man took my paperwork ( he scanned the photo and gave it back to use for AOS) and took my fingerprints (my hands were cold and they wouldn't scan for ages!)
Back to the main room whic by now was packed, people were standing everywhere! Waited for maybe 20/30 mins before my number came up again directing me to window 16 for the interview, nice friendly man asked a few simple questions (fiancées name, where/when we met, how many times we have met in person and where he lives)
I asked him if he wanted any of the small mountain of supporting evidence I had taken and he smiled at me and said actually you are approved :D
I doubt if I was at the window more than 2 mins and I was out of the embassy at 9
It really was as straightforward as everyone said it would be