Well, such interviews are getting more and more common, and not just for the unmarried. You will be asked a lot of questions about your relationship, and about your partner. Make sure you have a copy of your application in front of you. You are also expected to know about their family background - parents (their names), siblings etc. If and when you met them. Sometimes both of you are invited and seen separately, and your answers will be compared. All this is meant to weed out cases of non-genuine relationship.