Which approach is not permissible for demonstrating a witness's bias?

In legal proceedings, demonstrating a witness's bias is crucial for assessing the credibility of their testimony. One approach that is generally impermissible for this purpose is the presentation of external evidence directly. The rules of evidence typically restrict attorneys from introducing extrinsic evidence solely to prove a witness's bias. This is because bias is usually established through questioning during cross-examination, allowing attorneys to probe the witness's motives or interests without needing to present separate evidence that might detract from the jury's ability to weigh the witness's testimony in the context of the case at hand.

Highlighting a witness's past involvements, asking about possible motives during questioning, and using their testimony from prior cases can all serve legitimate purposes in revealing bias. These methods align with the direct examination and cross-examination practices that allow attorneys to explore a witness's potential motives or interests that could color their testimony. Presenting external evidence, however, could introduce issues that complicate the proceedings and may be seen as irrelevant or prejudicial, reinforcing the reason it's not an acceptable method for proving bias.