The question you're not supposed to ask is whatever the last question in your line of questioning would generally be. Or, I suppose: 'is the claim I've been ask

The question you're not supposed to ask is whatever the last question in your line of questioning would generally be. Or, I suppose: 'is the claim I've been asking you to provide evidence for true?'. In order to get a 'yes', you need to convince the witness of your claim. And most cross-examinations either involve discrediting the witness, or leading them to provide evidence that helps the side they're not on. So they're rather biased. If you ask it, and get a yes from the witness -- you almost certainly got enough from the witness for any judge/jury in the world to agree with you. So it doesn't really help. When you don't get a yes, you're opening the door to the witness providing a different explanation for the things you've established. Which you don't want. * In this case, Macleod had obtained everything he needed from the police officer to demonstrate that he had committed perjury. He could join the dots later, instead of asking the police officer to do so.