Very structured. Starts with approximately 45 minutes to type an essay in response to your choice of a few different prompts, all related to (fictitious) current events in international affairs. I was surprised at how potentially controversial all the scenarios were. The point seemed to be how well you wrote and argued your position though, and not on the position you chose.
The interview itself was with 3 people, at least 2 of which were former agents themselves. Very structured, which they said was for reasons of objectivity, with each asking a series of pre-written questions in turn. Each interviewer seemed to focus on a different "category" (school, work, exposure to foreign cultures). Each asked a lot of "can you tell me about a time that you..." questions. Very, very little feedback provided during the interview, other than occasional head nods, maybe half of a smirk, and LOTS of heads down scribbling copious notes. Just be confident that you're doing the best you can, because the interviewers are not going to provide any affirmative feedback. Knowing how to articulate how your past experiences demonstrates your ability to do the job seemed more important than the past experiences themselves.
The attitude, from talking to the agents after finding out I passed, is that they can teach almost anyone how to fire a weapon, drive a car, or get into shape. But they can't teach somehow to read and speak well, or to be a good person.