As an immigration lawyer, I often prep my clients for their marriage-based green card interviews. There's no guarantee of which questions you will be asked during the interview, aside from those on the actual immigration forms, but these are some of the questions that I ask my clients while preparing them for the interview:
The USCIS officer may ask you some or many of this type of question while you and your spouse are together. If he or she feels there is something that he/she wants to investigate further, you and your spouse may be separated. One of you will be interviewed first and all the answers will be written down by the officer. If you have an attorney with you, the attorney will stay with the spouse being interviewed.
Depending on the results of the interview, the officer can make the determination that your relationship needs to be further investigated. In that case, you will be scheduled for a second interview.
It is your right to have an attorney with you during the interview. The questions will still be directed at you and your spouse, but the attorney can give you simple guidance and/or make you feel more comfortable. In the rare case that the USCIS officer acts inappropriately, your lawyer can interject.
It is also important to have an attorney prepare your forms and your package for the USCIS. At the very least, you should have an attorney review your forms or answer any questions you have about specific items on the forms. Obtaining a green card is a major event in someone's life and should be given the appropriate attention and importance.