The first stage was a series of 3 30min interviews with senior research managers. They were all very similar, mainly asking about my CV, previous work experience, interest in the company. Each interviewer also went through a brief case study. They described a client project and asked what kind of experts I would suggest they speak to. All interviewers were open to questions and genuinely interested in getting to know you.
The second stage of the process was a full case study. It starts with a 10/15 minute phone conversation in which someone describes a case study to you and sets the task. You are then given 90 minutes to do your research and draft and email which serves as a response to the client. After that, the case study ends with another 15 min conversation in which you are asked to back your decisions and describe your research process.
The final stage is with the hiring manager and a member of senior management. This is another chance for them to get to know you and also for you to ask any questions you might have.
Overall, the process was clear although interview questions and case studies are challenging. You need to really think on your feet and be creative in how to approach them.