For the really good candidates (IE, ones who can answer basic programming questions and explain how a hash table works), I have an open-ended question which is really an open question in an active field. I really hoped I'd get candidates who would get that far (https://arxiv.org/abs/1309.2975) so that I could finally get some interesting answers I haven't heard before but most candidates I end up interviewing struggle to explain a hash table and what its advantages/disadvantages for counting unique k-mers are.
Or, if you want the candidate to talk about a certain approach, maybe think of some ways you can nudge them in that direction. If something's not at top of mind, some subtle hints from the interviewer could trigger a discussion of what they want to talk about. The interviewer shouldn't be tied to a "script".
