Here, straight from the Microsoft Jobs Blog, are some pitfalls to avoid:
As an interviewer, it’s extremely frustrating to be interviewing a candidate who answers a question before you’re done asking it. Not only am I now irritated that you’ve cut me off, but you’re likely going to give me an answer I didn’t ask for.
One thing I’ve done when interviewing, or am in a meeting, etc., is if I think of something I want to add or a question – I jot it down and save it for when it’s my turn to talk.
Be careful not to assume you know everything already. I’ve had a candidate interrupt just about everything I was saying as I was preparing him to interview here. He came across as impatient and that he felt I had nothing to offer him. It didn’t leave me with a good impression. Even if you do know it all, try not to let it show.