Familiarize yourself with the purpose and reasoning of the census. It is primarily for allocation of Representatives in the US House. It's secondary purpose is allocation of Federal government services based on population.
I have ZERO expertise in your field, but I can offer some general advice for your interview:
1) You want to exude confidence, competence, and come across as easy to work with. BODY LANGUAGE is HUGE. I suggest watching a few YouTube videos on body language, you could even google “body language in interviews.”
2) These days, job interviews are often conducted via a list of similar questions. You could also do an internet search for “interview questions.” Then you can practice what you would say if you get similar questions.
3) you could try rose playing the interview with a friend or family member, especially if you are nervous.
4) Dress in simple “business” attire regardless of the dress code for the position you are applying for. If you don’t own those kind of clothes, try a thrift store. I get most of my professional attire second hand.