Given the size of the organization, the interview will be based on what Bureau and office you have applied for. I interviewed for both a regional and functional Bureau. The difficulty of the interviews was comparable, and I felt it was much more relaxed than expected. I think the most important thing for State is to gauge your ability to act diplomatically under stress, verbal and written communication skills, and genuine interest in the area/function of the Bureau you're applying for. I would say that after having interned there both in person and virtually, the people there are generally relaxed, so it's much better to let your personality show instead of being overly formal.