For the chat application, I started by outlining the user requirements and features like real-time messaging, user authentication, and group chats. I discussed using WebSocket for real-time communication and a NoSQL database for storage due to its flexibility. The interviewer seemed engaged during this part, asking how I would handle scalability and ensure data consistency, which was interesting. I talked about partitioning the database and implementing eventual consistency, but I stumbled a bit on some technical detail about load balancing, so I wasn't completely convincing there.