My friend works at immigrations I have been asking her questions about things regarding sponsoring SPOUSAL (or common-law or conjugal) and she tells me they usually only have interviews if they have a reason to believe you relationship to sponsor is not sincere according to your application.
She said they usually only interview in extreme cases. She told me that they look for your reactions as well as your answers... as they sometimes ask you very rank questions as they want to observe you and see if you are really in a sincere relationship with your significant other.
She suggested to me to just show as much proof as possible (i.e photos, past phone bills, emails, etc.) and that it is very likely (if you have a sincere relationship) that they will call you in for an interview.