Present and past work history
The easiest place to start your questions is with their present or most recent job, and work backwards. Never start with their first job, as this could be some years ago and may not even be relevant. Depending on the work history you may not need to go through every job. Build your understanding of what they do by finding out about their company first, then their level of seniority and how they are managed, before moving on to specifics of their role. This will enable you to put their experience in context and understand it more clearly.
Questions about present and previous companies
To get the interview started ...