Generally speaking just make sure you're not caught flat footed by questions like "Why do you want to come to Japan", "What will you do when you come back", "How would you deal with *x situation which is challenging*" (usually: homesickness, intense culture fatigue/shock, not being able to speak the language fluently, inaka living etc) and just ensuring that you are telling the truth on your SoP/other stuff you've submitted.
Realistically you should be thinking about all of these questions anyway on the assumption that you'll be moving to a different country, so hopefully it won't be too hard coming up with answers.
Realistically you should be thinking about all of these questions anyway on the assumption that you'll be moving to a different country, so hopefully it won't be too hard coming up with answers.