The interviewer should say something like (in a friendly tone) "we're not trying to catch you out. Please do ask for clarification if I haven't been clear with

The interviewer should say something like (in a friendly tone) "we're not trying to catch you out. Please do ask for clarification if I haven't been clear with any of the questions", and start the interview with a discussion style question to allow a bit of chat back and forth between the candidate and interviewer, to tip it more into a conversation than an interrogation.