I myself have never had an interview but I can at least tell you how I am preparing. I would just know everything about you and your significant other. Some things I would make sure you know are:
- Age / birthdays
- How you guys met. If it was online make sure you know when you guys met online and how. Along with when was the first time you met in person. Location, date, place where you stayed during the visit.
- Friends and family members names.
- When did you get married / where you got married / location ect
Pretty much just do your best to know everything and anything about each other. As long as you know about each other very well you shouldn't be too worried.