The interview can be about anything. Officer may have concerns regarding the validity of your employment letter. So, try to gather as much documents as you can about your work. In Canada when a company is registered, they have article of corporation, they have yearly tax sheets and so on. See if your work is willing to provide you with those documents. If you were on the payroll, then you surely have paid your taxes. Gather anything you can get from the company about your income tax for the year you worked there and even the year after if you continued to work there. Update your employment letter if you are still an employee in that company.
Take some pictures from your work place so to show it actually exists. Did you take pictures with your colleagues at work? This will surely help. If you talked with the clients through Emails, copy some of them (black out sensitive information) to showcase your work. Basically, you want to show you were physically present at your job, you were being paid for it and also that this job really exists, so does the company.
Now, the interview may be related to completely different issue. Have you applied for your TRV to Canada before? If there are any discrepancies between your old applications and the new ones, it will surely raise the flag. Is there anything in your application that you should have mentioned but didn’t do so? The interview may be related to that as well. As you can see, unless you know why you are going there, the possibilities for the questions are limitless. The best advice I can give you is to take as much with you as possible, related to anything. Do not give the officer anything extra unless he/she asks for it. You do not want to create more question marks for him. Be calm but be truthful. I cannot stress this enough. Never, ever lie. Even if you think it may hurt your chances. Be upfront with him. Smile, don’t stress and it may be one of the best interview you have ever had in your life.