Went through a staffing agency, who will bring you in for an informal interview with them first. Assuming you get the interview at Mercer, they will ask you to go through your resume and experience, why you're interested, and what you know about Mercer. The interview questions come next, after which you should ask a few questions. 30-45 minutes, you'll usually hear the next day or so. Whole process is a couple of weeks.

Received an email from a recruiter. Had a phone interview couple days later, followed by Skype interview. On site interview a week after that. No difficult questions, very easy process.