Interviewers give signals. Learn to read them.
"Tell me more about X": They want depth here. Go deeper.
"That's fine, let's move on": You've spent enough time. Don't linger.
"What if X fails?": They want to hear about fault tolerance.
"How would you scale this?": They want horizontal scaling, sharding, caching.
Nodding and taking notes: Good sign. Keep going.
Frowning or silence: Pause and ask "Does this approach make sense, or should I consider alternatives?"