When approaching Science interview questions, you should have a general approach set out in your mind, one which you have practised and are confident in.
Your first step should always be to contextualise the question and consider what knowledge you can apply to it. Ensure that you consider what subject area you are being asked about. Questions are often very left-field and therefore you must be prepared to take a step back and assess.
Your next step should be to apply your A Level knowledge to the question as far as you can. Do not be concerned if this knowledge does little to answer the question.
Next, bring in any additional knowledge that you have gained from your general reading. Try to apply your wider knowledge as far as possible.
Finally, use logic and common sense to build out an answer, and fill in gaps in your knowledge using plausible theories and information from the tutor that you are talking to. Remember that this should be a back-and-forth discussion.