A non-citizen can often get a green card, changing their immigration status, when marrying a citizen. The government is wary, however, that some people may attempt to game the system by getting married specifically for the green card, not because they want to be married. As such, they can conduct interviews to decide if the marriage is legitimate.
The exact questions you face will be unique, but you can assume there will be some general relationship questions. They may ask what you did on your first date, how you met or how long you’ve been together. They’re just looking for the two of you to provide generally the same story. They could even ask who proposed or who brought up marriage first. You don’t always have to get every single detail correct, but if you say that you met at school and your spouse says you met online, you could have an issue.
Beyond the start of the relationship, they may ask you some questions about your general life together. These could be seemingly unimportant things, like who tends to get up first or which side of the bed you sleep on. They may ask you about the division of labor in the house, like who does the cooking and who does the dishes. Again, they’re just looking for you to generally agree so that they can see that you do live together and the relationship is real.
If this process feels a bit intimidating, it can absolutely help to work with an experienced law firm.