I had one phone interview with the hiring manager, and a week later I was invited to an all day in-house interview with 7 Amazonians. These were all 1:1 interviews that lasted about 45 minutes each. One of the Amazon interviewers is labeled the "bar-raiser" (this is usually the toughest interview) and the purpose of this interview is to make sure that they are not just hiring somebody who is good but somebody who is a superstar at what they do. The office enviroment is pretty casual and they even allow you to bring your dog to work. The people seem to be very sharp and serious about their work.