The interview process was pretty straight-forward, and in keeping with the expectation of any of the big consulting firms. The process was lengthy--many unofficial rounds of interviews between information sessions, "mock" case interviews, and informal networking events before having the opportunity to sit before the Principals/Partners.
The interview was a case format, two rounds, all portions in front of Director or Partner level practitioners. Both 30-minute cases were tailored to the government sector, and included both quantitative and qualitative analyses. The key is to know your stuff--that is, general frameworks, how the government agencies operate, current events, etc.
The first round includes a 30-minute case and a behavioral interview. The case is as explained above, the behavioral interview seems to gauge a) confidence, b) ability to "sell" yourself, c) interpersonal communication, d) cultural fit, and e) ambition. Basically, if you're excited about a career in consulting, know your resume, and can make connections, you'll knock this portion out of the park. My first round was on-site, but can also be in the form of a telephone or skype interview, from what I've been told by some of my peers.
The second round can be either the same day, or days, even weeks later. The time lag depends on the practice and position for which you're applying, but the interviews basically take the same form. A case, with a Partner, focusing on a government-specific "problem."
In sum, how well you do is up to you. Prepare. Prepare. Prepare. And bring a strong handshake and solid, natural eye contact. It's all basic person-to-person interaction, but pays dividends in having a good interview experience.