The way I see it, if they ask you a question, and you genuinely and enthusiastic talk about it and talk at length, keep going. The length should vary depending on your comfort with the answer. If an ethical question is asked, and you feel like you know a lot on the subject, talk at length, use tangents, bring up other ethics issues tied to it...I feel as if that would impress them. When they ask about volunteer activities, light up, talk about what you like most about it, show your enthusiasm through words and body language.
When they ask you something you aren't as confident in asking, answer what you can, and try not to ramble on with something you know little about. If you really don't know, say so, that will be just as impressive rather than fumbling and losing possesion of the interview.