Dear Probir bhai,
I applied for GCMS through my spouse's uncle who is a permanent resident of Canada. Any Citizen or permanent resident can apply for your GCMS on behalf of you provided that the documents are proper. If you have any relative or friend in Canada with at least PR status, youmay try that, and that is the easiest and cheapest way. You will get all forms inCIC website. It will cost CAD 5 DOLLAR ONLY. Please inform me if you need any further informtion.
Regarding your interview, first of all seat with a cool and activated brain. Gather all your documents that you have sent to CIC so far. Be sure NOT to provide any information that causes contradiction or discrepancy with the previous information. If your spouse would need to face the interview, prepare her in the same line. I think, this is the main issue that CIC wants to confirm.
Check with your employer/office whether there was any query from CIC. If not and if doable, than inform your office that there might be an enquiry from CIC regarding your job.
As you know, CIC selects interview candidates for two reasons; firstly, to clarify any unclear issues and secondly, on a random basis to ensure the quality of selection process. You may be from any group.
Please feel free to communicate any time. Wish you all the very best.
Rakib.