Hypothetical:
Two girls are fighting all-out in the hallway. A teacher is trying to break them up, but can't, and yells at the crowd gathering to help (because if there's a fight, there's someone watching). You have the opportunity and the ability to do so. Would you? Assume your relationship with these girls is neutral, you know them, you've spoken, but you haven't built up a rapport.
This actually happened to me a few days ago, and I didn't do anything. I feel kind of ashamed. I probably only would have made it worse, though.