Lay witnesses will often give you their opinions at depositions even when you didn't ask for them.
"The roof was in good repair."
"I wasn't at fault."
"I'm going to need more surgery."
"Our policy was perfectly reasonable."
Be alert to opinion testimony from lay witnesses. When you hear an opinion, get a list of all the facts upon which the opinion is based. Exhaust the witness's knowledge. Focus first on facts that can be verified with the witness's own senses ("I stepped on the roof and it was solid"), then move on to other sources upon which the witness is relying ("my doctor told me I'd need more surgery").
Separating fact from opinion is a technique that should be used with any type of witness. The lack of supporting factual testimony can be used later to further bolster a challenge to the opinion testimony by way of a motion or during cross-examination.
Partial source: "10 tips for effective depositions," by Thomas J. Vesper, Trial, August 2000. For more about depositions, see this weblog's "deposition category," as well as my deposition book, Deposition Checklists and Strategies (James Publishing 2006).