The interviewer was completely unprepared and unprofessional. Had I not been desperate for a job I would have turned them down. The workplace is very casual, which is nice.

It is best to apply directly by visiting their site. If selected for interview, it most likely means you have been deemed as a good candidate so you can only loose your job offer. You get one to one with HR then hiring team manager and possibly department director. It is important to appreciate the role you applied for and that you are yourself at the interview. You'll get more preparation by having a good night's sleep and not being late on the day that researching the company!