Interview could be for various reasons. I had issues with my marriage certificate registration so they had me called in just to ensure my relation was genuine and it was not all staged. Depending on the officer you get, they might be a little strict with questions but in the end if u are really genuine, you wont have any issue.
The major problem i see when files get transfered to local office is that you have no choice but to wait. Your file now is outside the processing time and u cant do anything but wait! Its the most horrible time ever.. for me, it took them 6 months for them to get back to me to schedule a relation interview. Once i did get my interview done, i received the AIP the same day via email. And next is my landing interview.
Total time... 2 years 4 months estimate.
Goodluck