The interview is meant to confirm or clarify some of the details that was provided in the sponsorship application and also get to understand your marriage/relationship better.........the visa officer during the interview may want the PA to discuss the genuineness of your marriage/relationship........so its very important that the PA be honest, confident, calm, and clearly discuss your marriage/relationship as it is in your current sponsorship application........if the visa officer is okay with the PA answers during the interview, the VO will proceed with processing the sponsorship application.
More evidence to proof marriage/relationship to sponsor is requested when you did not provide enough support documentation in your application........so review the checklist (Part B - 7...Proof of Relationship to Sponsor) for additional documents......you make provide letters from family and friends, social media post like on Facebook to show friends recognize your marriage, etc.
The PA is required to go with their travel passport when they attend the interview for identification purpose.