My interview was literally 1 minute, the longest part was waiting in line (at 4 different lines) and security checks.
The officer just looked at my file and asked me "so you're going to do a PhD in chemistry at X university?" to which I answered yes, and he replied "ok, your visa is approved"
but obviously your mileage my vary, usually phd ones are easier because you're fully funded, while for a master's they might look more into your financial situation
it also varies for different countries
so my advice is: be prepared for everything, but also be relaxed... save the stress for the interview at the airport ? (that scares the hell out of me)