Three windows: the first to submit paperwork. I forgot to hand in my "proof" of ongoing relationship and an engagement. The second window for fingerprints. The third for the "interview". Turns out they didn't need my "proof" - the CO said she believed me. I was done in 10 minutes flat. The wait overall was over 3 hours, but the actual interview was cheese.
I'd say it's true that their minds are made up before you even get there for the interview.
My advice? Just don't lie. Be up front, honest and open.