Initial phone interview was with an in-house recruiter. This conversation lasted less than 10 minutes, and I was asked to describe my background. They also asked if I was ok with traveling up to 80% of the time. They were also happy to answer my questions.
A day later there was a second phone interview with a ThoughtWorks consultant. The interview lasted one hour. This was a more in-depth interview about my experience as a UX designer. The tone was very friendly. I heard back from them later that day when I was asked to come to a group interview, what they call Super Saturday.
A week an a half later I participated in the one day group interview in their offices. I was asked to supply three references. There were almost 30 other candidates there, interviewing for several other types of developer, project management and business analyst positions. We were in the office for 8 hours. The whole morning was spent taking tests for logic, problem solving and then filling out a personality survey. The logic test was very hard. It was developed in-house and probably can't be prepared for. The two tests were extremely challenging. The afternoon was divided into three sessions: individual interviews, team activities and then an open group discussion on values. In this last discussion, the discussion leaders posed situations where ethical behavior is pitted against business needs and asked how people would respond. For instance, "Suppose we wanted to open an office in a country where bribing officials is common practice. Would you do business there?" It was a long, intense day.
There was a lot of emphasis, in both the individual interviews and the group interviews, on how one deals with clients. For instance, "What would you do if your client didn't like your idea and wanted to do things their own way, but you know your way is better?"
As far as preparing, it probably helps to be able to express your opinions or frame your experiences within the context of an Agile Development Process. Think about how you would actively include the client in the development process.
My impression was that this is a fun but also intense and demanding work culture. They seem to be looking for a very particular type of personality---quick-thinking, decisive and outspoken. They describe themselves as politically liberal and progressive.