Very long application process but easy enough if you're organised, it can be draining financially though (£75 for a mandtory TB check). The interview I attended was held at the Japanese Embassy in London - make sure to bring all required documents - Passport etc. (I was with another applicant who literally got kicked out for not bringing everything!) Easy process, get taken through to a room where you complete an English test - grammar, corrections, word meanings etc. You then sit with a previous JET and ask any questions. Afterwards you are accompanied to your interview. I was personally interviewed by one Japanese lady and one British former-JET.

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.