Interview could be formal or informal depending on the need for you. If they need you, you will probably get the job. You will probably have to demonstrate a little knowledge to be of use to them. Usually just one interview, if any. Hopefully some training to follow unless they really need people in the field.

You usually get a phone screen where the interviewer asks you questions on sedimentary rocks and the usual behavorial questions. But my interview was one on one. So the interviewer asks me my experience in geology and showed me some rocks to identify (sandstone, siltstone, shale, limestone). He described the position and asked me if I was willing to do that. Overall, it was not a difficult interview if you are prepared for it.