My interview process was composed of an initial phone interview, 5 or 6 face-to-face interviews (for which I was flown to the Pittsburgh office), and a couple of extra phone interviews.

These last two phone interviews were due to a confusion on their side: they needed to assert my knowledge of Java, but all interview responses ended up being done in Python. It took about 2.5 months from application to final offer.

Questions were all technical and not easy. The interviewer is there to help you, and you need to talk to them, even to fully understand the question being asked. Talk through your logic process; they need to understand it and can better assist you in case you are going the wrong route.

Unless you are really smart and very knowledgeable of CS (algorithms, data structures, analysis, etc.), you need to study, train, and prepare. Online courses, interview preparation books, and Wikipedia are very good reference material.

They ask for previous colleagues/managers for referrals and do call them.