I have had two interviews. Both were very different from each other. In the first, it was extremely well structures, and in the second it was more of a conversation -- the interviewer almost seemed like they were trying to sell me the job at times. The organizational structure allows a good deal of autonomoy for each researcher, so this is probably a factor behind interviews being varried. If you interview, the only way to get some preparation is to know a bit about who is interviewing you.