I exchanged emails for over a month with the Principal Investigator before the phone interview. In the emails, he inquired into my personal qualities extensively to determine my ability to stand up for myself, as he felt that the atmosphere of intellectual debate at the institute might be overwhelming to some. He sent me several journal articles before the phone interview and then questioned me about my understanding of their content in the interview, as well as my intentions for my future in science.

Just one round of interview with the PI and it is not very hard. You need to prove your ability to lead scientific projects alone and show creativity. You also need to show that you are a good person.