The interview process I participated in was on-site and took approximately three hours, including breaks and waiting for our own turn. I found that sonsulting other glassdoor.com reviews to be somewhat helpful, especially when preparing for the interview questions.
There were three components to the morning:
1) A 30 minute written multiple choice test comprising of 32 questions in the areas of a) vocabulary b) following directions c) problem solving and d) map reading. Personally I found the test easy but I also have a college education. A few people appeared to not make a passing score (must get 20 questions correct). It's not a test you could really study for, unless you were to obtain a mock test and answer key. We all didn't receive the same test, so even if you were to obtain and study off a copy of a test ahead of time, it would only be partially helpful. If one's understanding of English or reasoning ability is impaired, it's likely to be problematic. Otherwise, the written test is not particularly difficult.
2) The second portion of the interview process required us to engage in a mock interview. This was where we pretended we were on the job in an actual field interview. The Census employee pretended to be the person being interviewed. We met in a separate room together. There were ample instructions and it merely required us to follow a few very simple format and read from the booklet. I would suggest reading aloud in a clear voice, with a warm tonality. I found this to be the easiest and the most fun part of our morning.
3) The final portion of the interview was where we were called into a separate office to answer a few questions 1:1 with the Census employee. I've listed some of them in the interview question section below. (Questions are paraphrased.)
SPECIAL NOTE: There was particular emphasis on, in addition to the basic requirements, that applicants be available to 1) travel 2) work evening and weekend and other odd hours when interviewees would be available 3) have a lot of flexibility in these areas 4) be willing to be persuasive when met with resistance or initial unwillingness to participate (in other words, to "sell" people and 5) be open to doing field work with surveys which, in some cases, could take hours to complete.