STAR stands for Situation, Task, Action, Result. Every behavioral answer follows this structure.
Situation: Set the context in sentences max. "I was on a -person team building a payment service. We were weeks from launch."
Task: YOUR specific responsibility. Not the team's goal.
Action: What YOU did. Use "I", not "we." This is the longest part.
Result: Quantified outcome. "Reduced latency by " beats "It went well."
Keep the total answer under minutes. Practice with a timer.