the one thing that will really help is for u to know about ur wife or husand which ever case may apply inside out...thoroughly without having to think before answerin....another thing will be for u to know abt u too also without having to think b4 u answer...sound like a joke but am serious because i passed thro an interview myself.
Whether it will be tough or not depends on wat the immigration officer glean from the information u submitted in your application...go backto the copies of ur application and thoroughly go thro all the info u submitted...for me the officer asked me abt the story of me and my wife, sat back and listened....over 90% of wat she asked me was abt my wife...and I could see she had prepared her questions before coming...also if u have new document to further prove ur relationship(whatever u can lay ur hands on)please go with them. Its a point that can alter ur life for a long time and u need take it seriously...for me at the end of the interview which lasted abt 20minutes...the officer told me there point blank that she have accepted my marriage as real.i learn atime they(cic) will tell u that they will get back to u in 2 weeks.
Good luck
