I was just there a couple of weeks ago to get my UP visa reprinted (since they ripped it when they initially issued it and Immigration at Heathrow was not amused--although thank Goddess they let me through, they also told me to have it re-issued the next time I was in NYC).
You can make appointments up to two weeks in advance. I made a 9:00 appointment and showed up at about 8:45. At 8:50 they started calling out the names for 9AM appointments. They checked each person's passport against the master list before opening the door. They have 8 or 10 people scheduled in each appointment time slot so you will go up with a bunch of other folks.
Security is a bit tighter now as you are not allowed to take any cell phones inside. I didn't bring mine, but I think they just hold them downstairs until you come out.
Once inside, it is the same as it's always been...getting in the queue is a bit of a free-for-all and they open at 9 and start taking people. If you applied on the net make sure to print out your application. When I was there I was doing my business at the reception desk rather than in the visa queue, and people kept coming up to reception to have their internet applications printed out. Then they had to get on the queue again.
There is a passport photo machine inside the visa office just in case you forget to get them made. But I would have mine done in advance anyway.
If your visa application is straightforward and you are one of the first to get served, the whole thing could be over in as little as 20 minutes including the time it takes to pay (they send you to a separate queue to do that). You will then be asked to pick up your passport in an hour or two. I can't imagine that you wouldn't be able to leave and come back later--I did in October when I got my visa--but I would check with them first just to make sure.
I have a different visa than the one you are applying for so I'll let the married people answer your questions about evidence.
Emily
