Go through the application package you and your spouse originally sent in. Know everything inside and out. They can ask you anything that you wrote in your package, so make sure you have dates and names etc down pat. They can ask you anything they want to ask you relating to your package or not (if they ask stuff like what colour underwear does your spouse wear, you can answer any way you want becauase I highly doubt you would have put that in your application!!!) Also, they never ask to specifically interview your spouse, they ask to interview the person being sponsored so you don't have to worry about getting your answers to "match" - just make sure you know what is in your application package! Ofcourse if they have reason to believe your marriage isn't "bona fide", they will try to rattle you. But if you have nothing to hide, then you have nothing to worry about! Our interviewer was serious but light hearted as well, so there were no issues there. Very similar to the passport control guys at the airport. Be nice, be respectful, honest, and have a sense of humour and you will be alright. =)