Sharing my test/interview experience... my advise:
In preparation for the test: study (read) and try to understand each chapter of Discover Canada (at least) twice SLOWLY and listen to the MP3 (if you are an auditory learner). If you are a kinesthetic or tactile and visual learner, you should do some practice tests, however, nothing beats the book. You should read the book because you really want to know about 'your new country' and not simply to pass the test. In doing so, studying will be more meaningful and the experience will make it easier to adsorb the material.
The interview: you should assume that the agent doing the interview is the actual processing agent, so they know everything about you. Their intent is to confirm your identity (original docs), check your 'story' (work etc.), your language skills, to raise any concerns they might have etc. If you do not have enough buffer days, you should assume that the scrutiny will be greater. So, more than enough buffer days may save you some headaches. My interviewer did not even look at any other documents, e.g. COPR, diplomas, colored copies of passport pages (since my travels was somewhat extensive); kids' documents etc. However, the agent commended me for being well prepared. Overall, great interview experience. On the hand, I observed that 2 of the other interviewers were very thorough and strict (depending on the case and the applicants).
Be prepared!
P.S. For those with spouses, make sure the other party study the book and is well prepared. Unbeknownst to me, my wife wanted to beat my test score. Too bad... I beat her.