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."
Task: Your specific responsibility. "I owned the fraud detection module."
Action: What YOU did, not the team. Use "I" throughout.
Result: Quantified outcome. "Reduced false positives by %, saving \200$K per quarter."