I was there (IAD) a few weeks ago for my son's interview. There was a sign stating that they don't take walk-ins. There is a small waiting area outside of the office where you go to wait for your appointment. It looked like they would have 4-6 people scheduled for each time slot. An agent came out and called the time slot and we all got up to follow him in. Apparently, a couple of people didn't show and he asked if there were any people there that didn't have an appointment and wanted to do a walk-in. At the same time, there were a couple of people that had appointments for the next time slot but the agent wouldn't let them go early - he would only take those without appointments.

When I was in the office with my son, I heard another agent telling someone on the phone that they would take them if there was a cancellation for an appointment which matched up with what I had seen there in person. Accordingly, you might have luck by just showing up and hoping for a no-show.

Another data point for those that are looking to try and get an interview squeezed in with their wife/husband, etc... I was there with my two year old son and when the agent called his name we went up. The agent assumed that I was there for the interview and he asked if I also wanted to have them process my son at the same time. It appeared that as long as one of you has an appointment, they might be willing to let others in your same party squeeze in too.

Last piece of advice - check for new availability regularly. I initially had an appointment about 3 months out. Ended up moving it up several times as earlier slots opened, and finally ended up getting a slot for a few couple of days in advance of making the appointment. I also noticed they open up complete new days and times on occasion which creates a bunch of availability. Good luck!