You should have received an email stating the requirements. Typically, they look for relationship related documents. i.e. marriage certificate, the day you first met, wedding photos, honeymoon, outings, chat/call logs, emails if that's your way. They will ask specific details from each of these. They may ask for the description, how many people attended your wedding, honeymoon place and timeline, general photos of you two. Make sure you have clear answers and timeline ready to pitch at them. Don't get scared, but don't confuse them either. Be brief and maintain relevance. If they want to verify your relationship, you better answer to prove that you are right and authentic.
Also, my wife was asked to bring tax documents and payslips as we submitted in those in the application. She is working and we included those. They had minor confusion so they asked to bring documents related to it. She answered and clarified everything to them. It went well. We are at medical exam stage now.
Don't worry. They are very friendly people and if you are right and authentic, things will go smooth for sure. Usually, it takes about 15 mins for an interview.
