You don't need to go into overdrive like a lot of people say. What you need to do is be genuine about your passion and get it across by saying you read the news/keep up to date with legal developments. If your interested in a particular type of law then it always helps to state your interest, especially if the school offer a module in it. I think it's hard to fake genuine passion and if you try too hard your risk going the other way. Try striking a balance but remain keen enough that your interest shines through, but is realistic for your age and not embellished to massive lengths