In all interviews that involve any kind of problem solving, be it coding or system design or something else, the interviewer is looking to evaluate two main aspects -- knowledge and understanding.
- Knowledge can be demonstrated fairly easily, such as through the use of various data structures during coding interviews, or through the use of appropriate architecture components during system design interviews. At the same time, you aren't expected to know everything, so be open to admit that you're unsure about something.
- Demonstrating one's understanding of different concepts is equally (if not more) important. The only way to demonstrate this is by explaining the reasons behind your choices, by providing alternatives that you considered and by weighing the pros and cons of each approach.
Before you start solving any problem, it is crucial to first form a solid understanding of the problem itself. So make sure you ask questions and define the problem clearly.