"How have you overcome difficulties in the past"
"Explain what you did for your masters"
Then I was asked if I knew that the pay wasn't going to be too great.
Read through their papers carefully, and, if possible make a comment on their work. Just show, that you've been intersted enough to see what they work on.
Ask them, how many PhD students they have had up to date. Request to speak to the other members of the lab, if possible without the supervisor.