At our interviews, the visa officer asked the same questions over and over. For example, when she was talking to me (I am the sponsor) she kept asking me how I knew my husband would not just run away once he got to Canada. Unfortunately, the true answer is that no one could know that for sure - the visa officer's husband might have run off that very day, for all she knew. Not a good answer, though.
She kept asking my husband over and over why he wanted to marry a fat woman. His answers were perfectly fine, but the visa officer kept asking the same thing anyway.
Frankly, the best thing to prepare for the interview is to: 1. learn all you can about the sponsor's life in Canada (family, friends, previous relationships, jobs, hobbies, all about the town they live in, etc.); 2. make sure you look over the application and that your answers will be consistent with what is written there; and 3. realize that many visa officers will try to make you nervous or angry, so just keep calm, answer their questions, and don't get rattled or change your answers if they ask the same things over and over.