I can only comment on EWR:

Tried to walk-in on a Saturday afternoon with lots of time on my hand and expecting to be turned away. I explained that I had a Monday evening appointment but the Saturday would be more convenient. I was told that it was not possible as they were very busy and I should come back for my appointment. They did not explicitly say they don't accept walk-ins and had it been less busy with several no-shows they may have accommodated me (pure speculation of course). It doesn't hurt to try IMHO (with no expectations) if you are there anyway.

I then returned for my interview at the scheduled time and completed it.