I applied online. The process took 6 months. I interviewed at U.S. Department of State (Washington, DC) in Dec 2010
Interview
Internship interviews are very generic. More informational in nature. I've had two internship experiences and both calls are impromptu, with individuals from your potential office briefly discussing the basic functions of their particular office and asking questions pertaining to your functional understanding of the work.
Interview questions [1]
Question 1
One of my internships was in the Arms Control Bureau - my interviewer asked me a specific question pertaining an on-going treaty negotiation and if I had an idea of which particular country on the delegation was stalling negotiation efforts.