What should I expect if called back for an interview ?

I appeared for my F1 visa interview on the 2nd of May ,2018, and was issued a white paper. I went to the passport pick up location on the 7th of May, 2018 only to receive another White paper asking me to appear for another interview without asking for any documents or further information.

Why is this so?, and what should I expect ?

Thank you.

What is the form number on the white paper? Are you from Turkey, Egypt or Morocco? They do student CID verification on nationals of certain countries. They may not ask you anything.

Thank you very much for your swift response.
The form number is this- 2018120/291/3
I am from Nigeria.
Could it be that I have been put under CID check?.
Thank you.

I assume your passport is with Embassy. Two situations will cause further review - your passport has too many travel stamps OR it is brand new and has no travel stamps. Please keep us informed. Best wishes

Thank you.
Apparently, my passport was returned with the white paper letter saying “You are requested to schedule another interview”.