First round: group interview with other candidates. Each candidate is supposed to bring up a controversial issue or moral dilemma to discuss (you are told to think about this ahead of the interview). Example: what do you think about affirmative action for college admission?
Second round: another group interview round, a 1:1 "tell me about yourself" interview, a discussion with two company representatives where they give you a specific scenario and you argue for or against it (e.g. "In an election, should people be allowed to sell their votes to others?"). There is also a session where you listen to an audio recording about an employee having a discussion with some more senior people about her work, how she feels overwhelmed by it, and what can be done to improve things. Then you get 30 minutes 1:1 to ask questions about it and about the culture at Bridgewater in general.
The atmosphere was friendly, but I'm not a huge fan of the group interviews -- some candidates inevitably end up dominating the discussion.