The first round was largely informational with some behavioral questions. I certainly over prepared but I think doing so will be beneficial if you get asked back for another round as I did.

First they give you a phone or Skype interview that asks pretty basic questions about your experience. There are also questions that deal with why you are going into environmental organizing. The second round interview was held at a university and there was a morning and afternoon part. Groups are split up and half of the people do petitioning in either the morning or afternoon and the other half do in person interviews and public speaking. When I did it, public speaking consisted of three people in a group with two current Green Corp members and they gave you a dilemma, you had one minute to figure out how you'd respond and then I think three minutes to give a response.