I received both; a call from Foreign affairs as well as interview letters. They ask you questions in urdu only. Only one candidate told me that they had asked him about Yemen situation and put before him a sort of situation to know what he would do in that case. Rest were only asked about their qualification etc.
They asked me about my qualification, siblings, previous CSS attempts, temperament for accounts, etc. The whole interview was conducted in an amicable atmosphere. DG, sometimes agrees with you openly on some discussion. In the end, he wished me good luck. I thanked and came out.
I wore dress shirt with pent and tie.
To say thanks, you don't need to write thanks; just press thanks button at the bottom.