As I mentioned in "Controlling Your Client in a Deposition," I'm not shy about speaking up when a client starts volunteering information during a deposition. "Just answer the question, then stop," I might say. A related problem is when your client gives testimony you know is wrong. I'm not speaking about an outright lie, which happily doesn't seem to happen very often, but an unintentional misstep that might seem like a outright lie if and when the mistake is discovered later.
Examples are getting a date wrong or forgetting to name a witness when asked who else was present at the scene of the accident. It comes up most commonly when a witness is asked what he reviewed to prepare for the deposition and he forgets to mention an important document I showed him.
Back when I was a new lawyer, it was not always clear what to do when the client goofed and I knew it. Now I just speak up: "I think you meant to say 1997, not 1995" or "Wasn't Alice Bertran also there?" or "I also showed you the marketing brochure."
Is this coaching? I don't think so. To correct an unintentional error is not impeding the truth but precisely the opposite. And for what it's worth, I've never had opposing counsel complain when I've used this technique.