The interview process consisted of three parts: a short phone interview, a group interview at the Periscopix office, and a longer interview featuring myself and two senior staff members (again at the Merkle-Periscopix office).
The initial phone interview was fairly short, perhaps ten to fifteen minutes, and covered my education, my motivations, and several basic (but technical) PPC questions. The interviewer was very friendly, and I heard back from the company later the same day.
The second-stage group interview involved approx. ten people, and started off with Periscopix staff delivering a presentation about the company. Following this introduction and a brief chat, everyone delivered a short presentation about themselves (based on questions we'd been asked to prepare in advance). A few people were asked to stay behind after the interview had finished – I wasn't among those asked to stay behind, and I still received an offer, so don't read too much into this!
For the final interview, I was asked to prepare a presentation outlining how I would manage/structure the PPC account of a well-known retailer. It's essential to know your stuff in order to get through this stage (and indeed, to get through the interview process as a whole). The post-presentation questions I was asked were specific and technical, but the interviewers were both really pleasant and put me at ease – I got the impression that the key was to a.) demonstrate my knowledge, and b.) show that I could think on my feet.
All in all, I thought that the interviews were fair and well-organised. Everyone at Periscopix is smart, pleasant, and passionate about their job; this is reflected in the selection process. Interviewers will expect you to know why you want to work in PPC, to be a good cultural fit for the company, and to know a bit about the work you'll be doing (revise Google's help pages and read a few online PPC blogs, you'll be fine!).