The entire process was extremely quick and relatively painless. After submitting my resume through an employee referral, I was contacted over the phone and offered the opportunity to interview in-person the following morning. Since my resume came through at the tail-end of the hiring round, the entire interview process was extremely concise. Following a brief phone conversation, I came into the Boston office the next morning for 4 in-person interviews, which lasted a total of roughly 3 hours. Typically 2 of these interviews will involve case studies (i.e. market sizing questions or business situations) while the other two are behavioral-focused. I happened to meet with 2 senior analysts and 2 associate directors, although others have interviewed with anyone ranging from analysts to vice presidents.
The interviews were professional yet casual and somewhat laid-back, which accurately reflects the office culture in general. Case studies, while stressful, are treated as much as a conversation as a test. The idea is to show that you can talk your way through a problem, demonstrate logical problem solving skills, and think on your feet. As far as the behavioral interviews are concerned, these are primarily designed to determine how good of a "fit" you are for the company. In general, they are looking for someone who is friendly, engaged, and creative. Digitas is a place where self-starters and creative-types thrive, so it is important to show that you fit that mold.
After my interviews, I received an offer the following morning. Although experiences certainly differ from person to person, the entire process (from phone call to offer) lasted less than 48 hours. After going through several 3-4 round interviews at local consulting firms, this was an extremely pleasant process by comparison.