Impeachment attacks a witness’s credibility rather than the substance of their account. There are five conventional methods.

Prior inconsistent statements. The most common. In most systems the statement need not be shown to the witness first, but they must be given an opportunity to explain or deny at some point. Deposition transcripts make this straightforward: read the question and answer, page and line.

Bias, interest or motive. Employment, financial stake, family relationship, a pending claim, a fee arrangement for an expert. Frequently the most effective and the least used.

Capacity. Ability to perceive, recall or communicate — distance, lighting, intervening time, the sheer volume of similar transactions.

Character for untruthfulness. By reputation or opinion testimony, and by cross-examination on specific instances probative of truthfulness, without extrinsic evidence.

Prior convictions, subject to rules distinguishing crimes involving dishonesty from others and imposing time limits and balancing.

Contradiction by other evidence.

Collateral matters. Extrinsic evidence is generally not permitted to contradict a witness on a collateral point. Deciding what is collateral before committing to a line of cross is what separates effective impeachment from a detour.