The best scenario I had hear for getting involved was the "Shark Tank" scenario. For those that haven't heard it:
You're at an aquarium, Sea World, etc. and there is a shark tank. If a total stranger gets on the edge to take a picture and falls in, would you jump in to help save him? Now, what if your child fell in... of course you would.
Making the decision to get involved in a situation isn't always that easy. As a parent, say you see a teenaged girl being raped... as brother of somebody who was shot for his tennis shoes, what do you do when you see somebody being robbed at gun or knife point? As just a decent human being, I think when most of us see somebody being taken advantage of, we instinctively want to protect or fix the situation. Face it, most criminals are cowards and bullys. Half the time simply shouting at them will make them run off, but what if they pull a gun? It's a sad situation to be placed in, and seems unbelieveable to think that you would be put on trial for ridding the world of a scumbag criminal, but it will happen. If you do make the decision to get involved, be prepared for what happens after.
Best advice for if you do get involved, would be to spare no expense in getting the best attourney you can.