What questions to ask a famous mathematician
What advice would you give to a youngster interested in math? To a college student?
What do you do to overcome frustration when the going gets hard?
What sort of problems interest you now? 
Where should I look for interesting problems? 
What was your first interesting encounter with math? 


other things like that. 

Of course, the talk might cover some of this but might also suggest other questions. 

Of course, if the talk is being given to specialists, rather than the public, then questions specific to the talk would be appropriate, but possibly difficult to prepare in advance.