AS I wrote be4, your file 1st went to Missi.. to be finalized rt here in Canada but then they felt like sending to NDVO for minor doubts.
It is written in letter "approximately four to six weeks before the date scheduled" I read 1 guy got a month's notice thus you have sufficient time to prepare for the Interview. Even if they didn't ask for more docs; it does not hurt to take max available photos, up to date record of pone calls etc...be safe than sorry later in the room "I forgot at home" You are not allowed to take any electronic recording device with you. I think Cell phones can only be taken if SHUT Off the power? Not sure even any cell phone is allowed even shut off condition!
I don't think that you need further answers re; what they ask? Did you order GCMS notes which could help in details what is main concern? May be it is too late now as GCMS takes min of 30+ days.
Basically they have Qs about 1 or 2 things about your relationships. So be prepared both spouses. They pick up a word from application from imm5532 and ask Qs. If it differs they make a note, then ask U more and so on..Go thru what you actually wrote in PART C - INFORMATION ABOUT RELATIONSHIP. I am sure you or your agent have a copy of all forms sent. Again Honestly is the Best policy there without confused face. Answers should be given politely not angrily. Some times 1 has to speak bit more about relationships to impress. Ask anything here if still Qs
AND lastly it is nothing to worry too much as interviews are not conducted just to refuse but to clarify things.