Usually interviews are scheduled for two reasons 1) concerns around the relationship being genuine or 2) discrepancies / missing information / more information required based on the application content.
So, in preparation, gather additional proof of your relationship genuineness and review your application carefully for any possible discrepancies or "red flags" such as deviation from cultural n9orms in your marriage, affiliations, information that may have been missed. Be prepared to answer detailed and intimate questions about your relationship.
There is a thread in this forum for interview preparation - you should probably find it and review it.
C.