You'll arrive at a brownstone in Park Slope, and at first you may think this is the wrong place -- because a dog greats your at the door and it looks like it's a home. But 2 floors have been renovated to be offices (and the top 2 floors are still residences). Wear busy casual. VERY casual. Suit jacket not required. I think I wore a tie, and ended up taking it off at one point. You'll probably be interviewed by 2 to 4 people at once, and the topics can range all over the place. They're looking for your qualifications, but also how well you'd be able to fit into the group dynamics (since it's such a small office).
Definitely say nice things about their dog. Be friendly to everyone in the office (there's only like 12-15 people there), and everyone is bound to have an opinion.
You'll probably be given an editing test, so mentally prepare yourself for that. They are looking for grammar, spelling, inconsistencies. mathematical errors, logic errors, incorrect answers. all kinds of things. Take the test seriously, because some people they like do not pass the test.
You may be asked for a 2nd interview.
They're very pro-diversity on all aspects (race, gender, sexuality, trans-issues, etc) -- so if you're qualified that would not me an issue.