There was first an initial HR phone screening interview. This was very basic, just making sure you can make coherent sentences. The second step was a phone interview with my future manager; he was very interested in my education--what classes I took, and what I learned, as well as my previous work experience.
After passing this phase, I was invited to the office for a round of interviews with future colleagues. I was brought to one of the client conference rooms where I was interviewed, one by one, with more tenured Forresterites. They did not seem to have specific interviewer roles (almost like a good cop, bad cop), as some previous companies I've interview with did. Rather, each person asked whatever questions were on their mind, sometimes overlapping one another. It was very casual for the most part.
After passing this third phase, I was invited to participate in the final interview process--a presentation that tested time management, data interpretation, basic math, power point fluency and public speaking. I was fairly confident that I did well, and I received a job offer the next day.
Advice going in: Be honest. If you don't remember much from your statistics classes in college, say so. Just be prepared with a counter-point (i.e. quick learner--give example). Show enthusiasm toward your prospective role.
Good luck!