The best response, and the best way to fulfill your obligations as a concerned citizen (IMO) is to uphold the laws and principles we hold dear.
If you see a crime in progress:
1.) Call the cops
2.) Get a detailed visual description
3.) Use all your senses to identify threats to your own life, followed by the lives of your family, and then lastly to the lives of those around you
4.) Stay on the phone as long as is necessary to relay the situation in detail to the cops.
They are trained in getting to the "bottom" of the situation, including domestic disputes, unlike the humorous story related by Tanzer (ROFL!). Getting in the middle of a knock-down drag out might backfire. A little kid, getting stomped to death by a brute, or someone pouring gasoline on their Grandma is quite different. Anything short of imminent death, however, should keep you on that phone, singing like a Mockingbird.
http://www.youtube.com/watch?v=lRMfxzy9KlM (LANGUAGE)
The more info you give, the better equipped LE is going to be when they arrive, and (ultimately) the more likely justice will be served- which is ultimately what we all want.