Although these interviews happen, IMO they are not very common (first time I hear about this)...no need to worry...
Like @weirdestperson said, collect your thoughts and the documents needed, take additional evidence, make sure you present yourself in professional attire and show the respect they deserve, be confident, and also ask how much longer...don't show them that you are nervous, because nervousness is a sign of weakness...look them straight in the eye, 'yes sir/ma'am, no sir/ma'am,' and everything will work out...
At least you will be able to shed some light into the interview process, and you will be able to share it here for everyone to see...
Also, prepare yourself...luck is for the unprepared...success is for the people that are prepared...and 'guerra avisada no mata soldado (forwarned war does not kill soldiers)'