The questions vary from case to case. Basically, the questions will be about anything that was flagged as an issue by the visa officer. So look at the rejection letter and see what problems the visa officer mentioned. Also look at your GCMS notes, which will have more details about any problems. Then be prepared to explain why these are not serious problems, and gather any proof you can.
The immigration member hearing the case does not usually listen if you try to say that the visa officer lied or didn't understand something. Try to explain things in a way that does not contradict what the visa officer said the applicant said at the interview. However, if there is some cultural difference that caused the visa officer to misinterpret something, explain this.
They will also ask questions about anything in the file, so both the applicant and the sponsor should know the details of what was written in the application well.
