Don't mess up the application- even having papers in the wrong order or one item unsigned is an automatic application disqualification. When you go to the interview, be perky and energetic and focus on "grassroots internationalization", AKA just being a ridiculously happy person about sharing culture and language with people. Come with a few examples of how you'd like to do this. Don't worry about touting certifications or Japanese knowledge- you're here to assist teachers and 0 Japanese knowledge is expected (albeit make your Japanese level KNOWN, as it might affect your placement). Don't answer negatively about anything. But that's interviewing 101.

Smooth. Interviewers were kind. Asked questions about why I wanted to join, asked me some questions in Japanese, asked about ways I could contribute as an ALT, and that was that.