Most applicants aren't called for an interview - it's only needed when there are doubts about the relationship, which is usually the result of lacking proof of relationship or cultural norms of the applicant's home (sometimes for example they wonder about the relationship if a very religious person skips the religious ceremony and does a civil wedding instead).
There are some basic questions that she could be asked to prove that she does in fact know you well. Here's some in another thread:
http://www.canadavisa.com/canada-immigration-discussion-board/-t12122.0.html;msg46158And of course, depending on the reason why they asked for the interview, she would probably also be asked questions specific to your case.
And no, immigration consultants don't usually get to talk to immigration officers. At least not profesionally. They have experience with filling out the forms, so I guess that's where their knowledge of how to answer things comes from.
As for your "details" question... answer everything truthfuly. Don't neglect to mention something just because you can't prove it. Try not to give EVERY single detail, just to keep the amount of information managable - so for example, you wouldn't have to explain what the weather was like every day of your honeymoon or every single thing you ever did together. But try to include the details that do matter.
Most couples don't need a consultant to handle the forms for them. The questions are not that difficult, especially if you give yourself enough time to properly think them over. And when you're done, go over the application again and ask yourself if you have successfully represented your relationship and it's development. If not, you still have the option to put more on a separate sheet and explain anything else you think you couldn't cover in the forms.