Defending anything written on your CV

An interviewer picks an arbitrary line from your CV and asks you to talk about it. How should you prepare?

Assume every word on your CV is fair game and know it in depth. Only list techniques, projects and languages you can discuss in detail, including the maths behind them, what you personally did and what you would do differently. Interviewers use this to test honesty and depth; a vague answer on your own CV is a red flag, so trim anything you cannot defend.