'Ambush by deposition' is not as bad as it sounds.
It means "to catch a witness by surprise," which is the Perry Mason paradigm. Nothing wrong with a little curve, right?
It's not right to harass a witness with argumentative, irrelevant questions, but that's not what we're talking about. We're talking about pouncing on a witness with exactly the right question.
Before you schedule that ambush deposition, make sure you have the right prey. Not every witness is that important to a case, and it's not good practice to depose people who don't have anything to do with it.
Other forms of discovery may do the job anyway, like an interview, interrogatories or requests for admission. They are cheaper, and you don't need a sledgehammer to crack a nut.
But if you have the right witness in your sights, prepare your questions with an emphasis on getting to the critical point. Carefully guide the deponent like a wagon train to that pass of no return.
And here's the main point to setting up the ambush: you know what lies beyond. You have to know the answer to your question, and it cannot be good for the deponent.
Go for the Kill
I never killed anybody, but I saw a slaughter in a deposition once. The witness never saw it coming. The deposition video was awesome.
The deposing attorney had done deep research on the witness, digging up an article published in a small newspaper a decade earlier. Most people don't remember what they said that long ago, and neither did the witness.
After the deponent testified easily for an hour about critical facts, the lawyer asked him to confirm the statements he made in the article -- which completely contradicted his testimony.
A deposition transcript doesn't show how long it takes a witness to answer questions, but I think there were about 20 minutes of silence at the critical moment. The witness didn't die that day, but he settled for a lot of money soon after.