stroo wrote:It is not the monetary risk that would stop me from intervening. It is the question of whether I know enough to know who really is the party that needs protection. Some situations it may be clear. In many, it won't be.

Totally agree. For example, you see a man yell at a woman next to a van in a parking lot "Get in the car!" and then tries shoving her in. Well, this could be 2 different scenarios.
1. He's in the process of kidnapping her, where it would be justifiable to use force.
2. This could just be a spousal confrontation, where intervening to diffuse the situation could be called for as a good citizen (followed with a call to 911).
With regard to 3rd party defense, you just never really know with strangers. If you are familiar with the 3rd party, it's your call
Of course you can always intervene in a situation without having to use force. However, if the BG pulls a gun, all bets are off.