strategy of a maths/science chalk talk interview 1.Candidate is to teach 1 out of several topics given. Should there not be any preferrential topic to be taught in the eye of the interviewers? Is there any disadvantage not to choose the hardest topic?

2.In a real classroom, it's better to make the class interactive and ask students to come on the board to solve hands on problems. But as part of the interview, there's little time to do this. I can surely ask quick questions to the audience. How to balance between interactivity and time limit?
If you already know which topic is the most desirable one to the selection committee (perhaps, it's a subject they are not offering right now or willing to commit to a full-time instructor instead of a part-time one), I suggest choosing that topic. Also, if you know that your teaching material and/or approach for a specific topic is relatively unique and could be of interest to students (e.g., applied discussion of theories or real-world case studies), you might go that way. Other than those, I suggest sticking to the topic with which you're most comfortable.
I think Observing time limits during interviews is important and neglecting them could make you nervous. So engaging with the audience is OK as long as you don't waste time or enter into a heated discussion. 


Good luck with your interview.