Do the home office always interview couples to see if their marriage is genuine?

I'm marrying my girlfriend in the summer. I'm English and she is Brazilian here on a student visa. After we get married she's going to apply to have her visa changed to two year probationary visa. Then, if were still married Indefinite leave to remain and maybe pport. The application form asks where we met and when we decided to get married and why. Our relationship is genuine so I was just going to answer honetsly but is there anything that I should or shouldn't say and will there be an interview?