Tricky ones. Asking someone what happened or if they need help/consolation might depend on the circumstances and whether you know the person. It's tricky because some people might feel you were being nosy or interfering and object!

I don't really do violence, but I have got verbally cross occasionally.

I'd probably try to expose a thief, but again it would depend on the circumstances. I challenged someone in a shop once and he just told me to mind my own business. The next time I saw someone shoplifting, I reported it to security.