As a general interview process as happened in my interview, you are supposed to give a talk to the group and show your expertise in the field. Depending on the group, the candidate talks to the students and postdocs and the professor may or may not ask some of them what they think about the candidate. For most groups, if you are invited to give a talk, you are more than 50% of the time like to have got the job. Also most often you are supposed to find some overlap between your expertise and the professor's project interest beforehand, which is reshaped by the professor in time.