Generally, there are 2 interviews, a group and then an interview with both a school manager and an area manager. There 8 days of unpaid training at the beginning, which seems like part of the interview process, since they may but people.
It can be very fast-paced and overwhelming in the beginning, but after a while, one gets used to it. This is one of the reasons not a lot of people work too long at Berlitz.
It depends on what region you work, but generally, business attire (but you won't be able to afford it on an instructor's salary)