When opposing counsel asks you questions, it's called cross examination, or cross. You should always know what your client's attorney is going to ask on direct, but you never know what the opposing counsel is going to ask on cross. I think you should answer the opposing counsel's questions without stalling, but, don't get in a hurry . If you need time to think about the answer, then take that time. Before you answer any question take a few seconds to give your client's attorney a chance to object to the question.
Most witnesses have a problem with keeping their mouth shut.
They don't know when to stop talking, and this can really get you into trouble. When a simple yes or no will suffice, don't try to add the Gettysburg address to your answer. Answer the question, then wait for his next question. Don't try to play games with the opposing counsel. Never underestimate the opposing counsel.