No doubt, you gave your client strict instructions during your deposition preparations: Answer only the question that's asked; don't volunteer information; after you've answered the question, stop; if no one is talking, keep your mouth shut, etc.
So what do you do if your client, despite your preparations, turns out to be a non-stop talker? Repeat after me: You're a lawyer, not a potted plant. Speak up. Sure, it's the other guy's deposition, but if your client isn't following your directions, it's time for some reminders, such as:
"That's not the question, Bertha. Listen to the question."
"You've answered the question, Bertha. After you've answered the question, you should stop."
"Slow down, Bertha. I don't think you're listening to the question."
And so on. Because some clients are extra sensitive, I remind them several times before the deposition that I am the good guy, and the lawyer on the other side of the table is the bad guy, no matter how nice and friendly he might seem. I also tell the client to remember I'm the good guy if I have to get on them during the deposition. It's not because I'm rude, I say, but because I want you to stay on track.
Will the other side object to your instructions to your client to "listen to the question" and so forth? I've never had it happen. Opposing counsel generally wants your client to stay focused, and knows how to ask a question directly if he thinks you've interrupted an answer in process. All he's got to do is ask, "What were you saying before your attorney interrupted?"
The client must answer, but he's also been warned by you to be more careful the next time.