First stage of the application process was a Kenexa mathematical reasoning test, after which I was invited for two telephone interviews. Both interviews are competency based, keep in mind the Amazon Principles. Once you answer a question, they will ask 2-3 follow-up questions and dig deep into what you learned from that situation, or what happened afterwards, how other people reacted etc. so be prepared to know the situation in detail. The interviewers were both very friendly. I thought I did well, but they said at this stage their policy is to not offer any feedback, which is understandable given that they probably have a very large volume of applications. In hindsight my advice would be to practice your answer over and over and even think of any questions that could be asked about your answers that may catch you off-guard. I wouldn't say the interview is very difficult, but I would definitely recommend really thorough preparation.